4IO FISHES CHAP. 



The eggs are therefore described as " alecithal," and the segmenta- 

 tion as being " holoblastic " and " equal." On the other hand, all 

 Fishes possess " telolecithal " eggs, that is, ova in which the food- 

 yolk is more or less abundant, and tends to accumulate at one 

 pole of the egg (" vegetative pole "), while the opposite or " animal 

 pole " consists of protoplasm, comparatively free from yolk 

 granules and containing the nucleus (Fig. 234, A) The term 

 telolecithal is, however, a somewhat comprehensive one, and 

 covers important variations in the relations of the inert food- 

 yolk and the living protoplasm in different Fishes, which greatly 

 modify the process of segmentation. Thus there are some Fishes 

 in which the amount of food-yolk at the vegetative pole is 

 sufficient to retard segmentation in that part of the egg without 

 actually preventing it, and consequently segmentation begins in 

 the animal pole, and takes place more rapidly there than it does 

 when it extends into the vegetative pole. Hence it follows that 

 although the entire egg is segmented the blastomeres are of 

 unequal size, the animal pole giving rise to a large number of 

 small cells or micromeres, and the vegetative pole to a smaller 

 number of much larger cells or macromeres. The segmentation 

 of such an egg is said to be holoblastic but unequal (Fig. 234, 

 B and C). This type of egg is characteristic of the Chondrostei, 

 the Holostei, and the Dipnoi. In other Fishes, like the Elasmo- 

 branchs and the Teleostei, the food-yolk so greatly preponderates 

 that it entirely prevents segmentation in the vegetative part 

 of the egg, and segmentation is restricted to the small mass of 

 protoplasm (germinal disc) at the animal pole, in which the 

 nucleus is situated (Fig. 234, D). Eggs undergoing partial 

 segmentation in this way are termed " meroblastic." No hard 

 and fast line can be drawn between the two types, and in the 

 Chondrostei and Holostei an interesting transition between the 

 holoblastic and meroblastic ova may be observed. The egg- 

 membranes are formed either by the egg itself or by the 

 epithelium of the ovarian ovisacs, and, as will shortly be seen, 

 the character of the outer egg-membrane greatly influences the 

 mode of deposition of the eggs and their location afterwards, 

 In Elasmobranchs the egg is enclosed in a stout horny egg-shell, 

 secreted by the oviducal shell gland. 1 In many Fishes, as in 

 the Chondrostei, Holostei, and Teleostei, the egg -membranes 



1 See Chapter XVII. 



