410 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 unportant variations in the relations of the imert 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. 254, 
Band CC). This type of egg is characteristic of the Chondrostei, 
the Holostei, and the Dipnoi. In other Fishes, ike 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 
oc 
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.’ In many Fishes, as in 
the Chondrostei, Holostei, and Teleostei, the egg-membranes 
! See Chapter XVII. 
