942 



SPECIAL PHYSIOLOGY. 



Birds, and cff the Monotrematous order of the Mammalia. The ova of 

 the Amphibia are imperfectly meroblastic. In the second case, either 

 a very slight part, or no portion, of the yolk is nutritive, but all, or 

 almost all, is directly formative ; the yolk is comparatively small, fre- 

 quently clear, and less rich in granular organic contents. These ova 

 are called holollastic (oAo?, the whole). They are met with in the 

 eggs of the Echinodermata and of the Annelids, in those of the sim- 

 plest Crustacea and Arachnida, in those of Insects, and of the Mollusca 

 generally (excepting the Cephalopods), in the Cyclostomatous Fishes, 

 and, lastly, in the Mammalia, including Man. 



The holoblastic ovum (Fig. 116) consists of a transparent, homoge- 

 neous, or structureless vitelline membrane, which, together with a clear 

 outer stratum of the yolk, sometimes of considerable proportionate 

 thickness, constitutes the zona pellucida. Within this, and completely 

 filling it, is the limpid, or faintly granular germ-yolk, or formative yolk, 

 with its germinal vesicle and spot. The meroblastic ovum (Fig. 119) 

 consists externally of the vitelline or vitellary membrane, which is 

 thin, and often also homogeneous or structureless, but, in some cases, 

 slightly granular, or, in parts, indistinctly fibrous. There is no zona 

 pellucida, but the interior of the vitelline membrane is lined with a 

 stratum of polygonal nucleated cells, known as the epithelial layer. 

 Within this is the distinctly granular, nutritive yolk, a, which may 

 either be whitish or yellowish. On one part of the surface of the nu- 

 tritive yolk is a small circular disc, known as the cicatricula or germ- 

 inal disc. This is in fact the germ-yolk, or formative yolk, spread out, 

 in the meroblastic ovum, upon a small part of the surface of the nutri- 

 tive or food-yolk, instead of being spherical, and occupying the entire 



Fig. 116. 



Fig. 116. HoloblaBtlc ovum or germ-cell of a Mammalian animal, unfertilized. (Allen Thomson.) a. vi- 

 telline membrane or envelope, thick, and clear, afterwards forming the zona pellucida; within it is the 

 granular yolk or ctll contents; in this, the germinal vesicle or nucleus ; and in this, the germinal spot or 

 nucleolus. b, the ovnm, or germ-cell, burst, part of the granular yolk, with the germinal vesicle and its 

 contained germinal spot having escaped, c, the germinal vesicle surrounded by a little granular matter. 

 Magnified 80 diameters. 



vitelline cavity, as in the holoblastic ovum. Lying at one time in the 

 midst of the formative or germ-yolk, or germinal disc, are found, as in 

 the other ova, the germinal vesical and spot (Fig. 117). 



In the Mammalia the yolk is so small in quantity as quickly to be- 



