8 STRUCTURE OF THE OVARIAN OVUM. 



nucleus, which was termed by its discoverer, Purkinje, the germinal vesicle. 1 This, 

 which is about ? -^th inch in diameter, has all the characters of the nucleus of a 

 cell. It consists of a nuclear membrane enclosing a clear material or matrix, 

 embedded within which may be seen strands of karyoplasm, enclosing one or more 

 well-marked nucleoli (fig. 6, gv). Frequently there is but one nucleolus, which is 

 then large and prominent, and has received the name of germinal spot (macula ger- 

 minativa, Wagner, 1835). 



There is some doubt whether, before fertilization, there is another membrane (vitelline 

 membrane) enclosing 1 the vitellus within the zona radiata. The evidence of the presence oi 

 such a membrane is by no means clear, although its existence has been maintained by very 

 competent observers (v. Beneden, Balfour). 



The mammalian ovum (that of monotremes alone excepted) differs from that of other 

 vertebrates in the relatively small amount of nutritive material (yolk granules, deutoplasm) 

 which is embedded in its protoplasm. In fishes, amphibia, reptiles, and especially in birds, 

 the amount of such nutritive material is vastly greater than that of the protoplasm itself, so 

 that the very existence of the latter is obscured in most parts of the ovum, and it is only in 

 the immediate neighbourhood of the germinal vesicle that the protoplasm can be distinctly 



Fig. 7. DIAGRAM OP A HOLOBLASTIC (ALECITHAL) OVUM (A) AND OP A MEROBLASTIC (TELOLECITHAL) 



OVUM (B). (E. A. S.) 



Only a small part of the latter is represented. The yolk or food material is represented in both by 

 clear globules, which in B are seen vastly to preponderate, except in the immediate neighbourhood of 

 the germinal vesicle. 



recognized (fig. 7. B). It is here also that, after fertilization, the more active changes in the 

 ovum occur, and it is this part alone in which in the bird and most other oviparous vertebrates 

 the process of division or segmentation of the yolk and consequent formation of embryonic cells 

 proceeds. Hence these ova are said to undergo a process of incomplete segmentation, only a 

 part of the ovum appearing to undergo development, and they are accordingly termed mero- 

 blastic to distinguish them from those (like the mammalian ova) in which the yolk or nutri- 

 tive material is everywhere in relatively small proportion to the protoplasm, the whole of 

 which undergoes division after fertilization, and participates in the formation of the embryo 

 (Jioloblastio ova). This small amount of nutritive material in the mammal is obviously 

 related to the fact that the mammalian ovum early acquires an attachment to the maternal 

 system from which it is then able directly to derive its nutriment, whereas the meroblastic 

 ovum of oviparous vertebrata necessarily contains all the nutriment required by the developing 

 bird, reptile, or fish, until it is sufficiently advanced in development to emerge from the egg 

 and obtain food independently. Although, however, the mammalian ovum is holoblastic, it 

 is none the less clear, from a comparison of the early stages of its development with that of 

 the bird, that the ancestors of the mammalia must have had ova of the meroblastic type. 



Balfour has further conveniently distinguished between those ova in which there is a great 

 accumulation of nutritive or yolk material at one pole (tclolecitlial ova, as in the bird, 

 reptile, and fish amongst vertebrates), those in which the accumulation of yolk is in the middle 

 of the ovum (ccntrolecithal ova, as in arthropods), and those in which it is scattered pretty 

 equally in small amount throughout the protoplasm without any very marked accumulation 



1 Purkinje discovered the germinal vesicle in the bird's ovum in 1825 ; that of mammals was first 

 noticed by Coste in 1833. 



