674 



THE OVUM AND BLASTODERM. 



the greater part of the yolk is immediate!)' engaged in the first or pre- 

 liminary changes which precede the occurrence of embryonic develop- 

 ment. The whole yolk-mass, or its greater part, is therefore directly 

 formative or germinal, or, as it has been said, the ovum is hoJohJastk. 



But in birds and reptiles, in which the ova are comparatively large, 

 the greater part of the yolk forming the yellow yolk substance, takes no 

 immediate part in the first formative processes, and these are restricted 

 to the small whitish flat disc, called, the cicatricula in the fowl's Qgg, 

 which is composed of fine granular protoplasm, occupies a determinate 

 place on the surface of the larger yolk-mass, and, so long as the yolk 

 remains in the ovary, has the germinal vesicle situated in its centre. 



Fig. 4S9. — Ovarian Ovuji of a Maji- 

 Fig. 489. MIFEK. -f 



a, The entire ovum, viewed under 

 pressure ; tlie granular cells have been 

 removed from the outer surface, the 

 germinal vesicle is seen in the yolk sub- 

 stance within ; h, the external coat or 

 zona burst by increased pressure, the 

 yolk protoplasm and the germinal vesicle 

 having escaped from within ; c, germinal 

 vesicle more freed from the yolk sub- 

 stance. In all of them the macula is 



r- ' . ' ? To the part thus distinguish- 



'■-'' t^, ..<>"'' f^ble from the rest of the yolk 



■^ the name of germ may be given ; 



and it has also been styled the 



primary or germinal or formative yoUc, and the protoplasm or protoblast, 



while the remainder of the yolk-substance has been called the nutritive 



or food yoUc, the secondary yolk or deutoplasm. The oviparous ovum 



has therefore been named merodlasfic, or partially germinal. 



It is not known whether in the mammals' ovum the whole yolk ought 

 to be considered as purely germinal, or whether, as seems more pro- 

 bable, some nutritive yolk may not be combined intimately with the 

 germinal substance ; but even if so, it is obvious that the germinal bears 

 a much larger proportion to the nutritive yolk than in tlie bird or reptile, 

 and, as will appear more clearly hereafter, there is thus some founda- 

 tion for the distinction between the holoblastic and the meroblastic 

 forms of ova, although it may be that in different animals these forms 

 pass insensibly into one another. 



In both kinds of ova, however, whether holoblastic or meroblastic, the 

 su1)sequcnt phenomena of development show that the spot where the 

 organising process begins, occupies a determinate situation in the 

 ovum, and that the first rudiments of the embryo arrange themselves 

 in a determinate order round a central point in the germ. 



There is, therefore, in the ova of birds and mammals, a part of the 

 yolk which is more immediately germinal, and a central point of that 

 germ from which development spreads, to which the name of f/erminal 

 pole may be given. The centre of the germ is probably coincident 

 witli the ])lace last occupied by the germinal vesicle. 



Disappea-rance of the germinal vesicle. — The most marked 

 change in the interior of the ovum which is known to accompany its 

 maturation and escape from the ovary is the disappearance of the 



