THE SEGMENTATION OF THE OVUM. 



the midst of the cells of the blastoderm, but very soon its floor of cells 

 vanishes, and it lies between the yolk and the blastoderm. (Fig. 47 A.) Its 

 subsequent history is given in a future Chapter. 



Segmentation proceeds in Osseous Fishes in nearly the same manner as 

 in Elasmobranchii. In some cases the germinal disc is small as compared 

 with the yolk, in other cases it is almost as large as the yolk. The only 

 points which deserve special notice are the following : (i) Nuclei, precisely 

 similar to those in the Elasmobranch yolk, appear in the protoplasmic 

 matter around the germinal disc ; (2) After the deposition of the ova there is 

 present in some forms a network of protoplasm extending from the germinal 

 disc through the yolk 1 . At impregnation this withdraws itself from the 

 yolk. It is to be compared to the protoplasmic network of the Elasmo- 

 branch ovum. 



A 



FlG. 47. TWO LONGITUDINAL SECTIONS OF THE BLASTODERM OF A PRISTIURUS 

 EMBRYO AT STAGES PRIOR TO THE FORMATION OF THE MEDULLARY GROOVE. 



ep. epiblast; //.lower layer cells; m. mesoblast; hy. hypoblast; ^.segmentation 

 cavity ; es. embryo swelling ; n'. nuclei of yolk ; er. embryonic rim. 



There are two types of meroblastic ova. In one of these 

 (Aves, Elasmobranchii) the germinal disc is formed in the 

 ovarian ovum. In the second type the germinal disc is formed 

 after impregnation by a concentration of the protoplasm at one 

 pole. This concentration is analogous to what has already been 

 described for Anodon and other Molluscan ova (p. 100). 



The ova of some Teleostei are intermediate between the two 

 types. 



The ovum of the wood-louse, Oniscus murarius 2 , may be taken as an 

 example of the second type of meroblastic ovum. In this egg development 

 commences by the appearance of a small clear mass with numerous 

 transparent vesicles. This mass is the protoplasm which has become 



1 Vide Bambeke, loc. cit. 



2 Vide Bobretzky, Zeitschrift fur wiss. Zoologie, Vol. XXIV., 1874. 



