Book III chapter 3

The class of fishes is also oviparous. Those among them which have the uterus low down lay an imperfect egg for the reason previously¹ given,’ but the so-called ‘selache’ or cartilaginous fishes produce a perfect egg within themselves but are externally viviparous except one which they call the ‘frog’; this alone lays a perfect egg externally. The reason is the nature of its body, for its head is many times as large as the rest of the body and is spiny² and very rough. This is also why it does not receive its young again within itself nor produce them alive to begin with, for as the size and roughness of the head prevents their entering so it would prevent their exit. And while the egg of the cartilaginous fishes is soft-shelled (for they cannot harden and dry its circumference³, being colder than birds), the egg of the frog-fish alone is solid and firm to protect it outside, but those of the rest are of a moist and soft nature, for they are sheltered within and by the body of the mother.

The young are produced from the egg in the same way both with those externally perfected (the frog-fishes) and those internally, and the process in these eggs is partly similar to, partly different from that in birds’ eggs. In the first place they have not the second umbilicus which runs to the chorion under the surrounding shell. The reason of this is that they have not the surrounding shell, for it is no use to them since the mother shelters them, and the shell is a protection to the eggs against external injury between laying and hatching out. Secondly⁴, the process in these also begins on the surface of the egg but not where it is attached to the uterus, as in birds, for the chick is developed from the sharp end and that is where the egg was attached. The reason is that the egg of birds is separated from the uterus before it is perfected, but in most though not all cartilaginous fishes the egg is still attached to the uterus when perfect. While the young develops upon the surface the egg is consumed by it just as in birds and the other animals detached from the uterus, and at last the umbilicus of the now perfect fish is left attached to the uterus. The like is the case with all those whose eggs are detached from the uterus, for in some of them the egg is so detached when it is perfect.

The question may be asked why the development of birds and cartilaginous fishes differs in this respect. The reason is that in birds the white and yolk⁵ are separate, but fish eggs are one-coloured, the corresponding matter being completely mixed, so that there is nothing to stop the first principle being at the opposite end, for the egg is of the same nature both at the point of attachment⁶ and at the opposite end, and it is easy to draw the nourishment⁷ from the uterus by passages running from this principle. This is plain in the eggs which are not detached, for in some of the cartilaginous fish the egg is not detached from the uterus, but is still connected with it as it comes downwards⁸ with a view to the production of the young alive; in these the young fish when perfected is still connected by the umbilicus to the uterus when the egg has been consumed. From this it is clear that previously also, while the egg was still round the young, the passages ran to the uterus. This happens as we have said in the ‘smooth hound’.

In these respects and for the reasons given the development of cartilaginous fishes differs from that of birds, but otherwise it takes place in the same way. For they have the one umbilicus in like manner as that of birds connecting with the yolk — only in these fishes it connects with the whole egg (for it is not divided into white and yolk but all one-coloured) — and get their nourishment from this, and as it is being consumed the flesh in like manner encroaches upon and grows round it.

Such is the process of development in those fish that produce a perfect egg within themselves but are externally viviparous.

1 previously   

adv.以前，先前（地）

参考例句：

• The bicycle tyre blew out at a previously damaged point.自行车胎在以前损坏过的地方又爆开了。
• Let me digress for a moment and explain what had happened previously.让我岔开一会儿，解释原先发生了什么。

2 spiny   

adj.多刺的，刺状的；n.多刺的东西

参考例句：

• This is the Asiatic ornamental shrub with spiny branches and pink blossoms.这就是亚洲的一种观赏灌木，具有多刺的枝和粉红色的花。
• Stay away from a spiny cactus.远离多刺仙人掌。

3 circumference   

n.圆周，周长，圆周线

参考例句：

• It's a mile round the circumference of the field.运动场周长一英里。
• The diameter and the circumference of a circle correlate.圆的直径与圆周有相互关系。

4 secondly   

adv.第二，其次

参考例句：

• Secondly,use your own head and present your point of view.第二，动脑筋提出自己的见解。
• Secondly it is necessary to define the applied load.其次，需要确定所作用的载荷。

5 yolk   

n.蛋黄，卵黄

参考例句：

• This dish would be more delicious with some yolk powder.加点蛋黄粉，这道菜就会更好吃。
• Egg yolk serves as the emulsifying agent in salad dressing.在色拉调味时，蛋黄能作为乳化剂。

6 attachment   

n.附属物，附件；依恋；依附

参考例句：

• She has a great attachment to her sister.她十分依恋她的姐姐。
• She's on attachment to the Ministry of Defense.她现在隶属于国防部。

7 nourishment   

n.食物，营养品；营养情况

参考例句：

• Lack of proper nourishment reduces their power to resist disease.营养不良降低了他们抵抗疾病的能力。
• He ventured that plants draw part of their nourishment from the air.他大胆提出植物从空气中吸收部分养分的观点。

8 downwards   

adj./adv.向下的（地），下行的（地）

参考例句：

• He lay face downwards on his bed.他脸向下伏在床上。
• As the river flows downwards,it widens.这条河愈到下游愈宽。