SUBSEQUENT TO CLEAVING. 77 



in those invertebrate animals, such as certain Ascarides and other 

 Entozoa, in which the whole yolk is included in the cleaving or other 

 process which ensues shortly after fecundation, the embryo is at once 

 formed out of the entire mass of particles into which the yolk is re- 

 solved at the completion of this process. And in those, such as the 

 Sepia, in which the process of division is confined to a limited part of the 

 surface of the yolk, the only notable change which ensues between the 

 completion of this process and the first appearance of the embryo, is the 

 arrangement of the ultimate segments at the surface of the yolk into a 

 double membrane, which is analogous to the germinal membrane in the 

 ova of other animals. The inner layer of this membrane constitutes the 

 inner and outer yolk-sac, while from the outer layer is produced the 

 embryo and its several organs.* 



In amphibia and fishes, however, certain other changes occur during 

 this period which are deserving of notice. 



The most important contribution of late years to this portion of the 

 subject of development, has been furnished by Vogt.f In his opinion, 

 the formation of cells in the yolk does not commence until after the 

 completion of the cleaving process. When this process has entirely ceased 

 and the ovum of Alytes has regained its former smooth exterior, the 

 central part or yolk-nucleus (Dotterkern) is observed to have a darkish 

 yellow colour, while the external cortical part, which is composed of 

 molecular corpuscules, among which are scattered a few free germinal cells 

 or vesicles (see page 63), has a whitish aspect. After a period of apparent 

 rest I succeeding the completion of the cleaving process of the yolk, the 

 development of cells commences at that part of the cortical portion of 

 the ovum at which the embryo subsequently appears, whence it extends over 

 the remaining surface of the yolk and towards the interior. This forma- 

 tion of cells, according to Vogt's account, takes place by the production of 

 an enveloping membrane around each of the germinal cells or vesicles 

 already existing in the cortical substance. A portion of the granular ma- 

 terial of the ovum is included between each germinal cell and the newly- 

 formed investing cell-membrane, and constitutes the nutritive contents of 

 the yolk-cell. It would seem, also, that new germinal cells are being con- 

 tinually formed, and then, by the above process, developed into yolk-cells. 

 The first set of yolk-cells which are produced, accumulate at the surface 

 of the ovum, and adhere together so as to form a membranous layer, 

 which gradually extends over the entire surface, and increases in thickness 

 as fresh quantities of cells are deposited on its interior. 



Within the substance of this membrane is subsequently developed the 

 earliest trace of the embryo, namely, the primitive groove, bounded by 



* Kolliker, Entwickelungs-geschichte der Cephalopoden, p. 61. 



t Entwickelungs-geschichte des Geburtshelferkrbte, and Histoire Naturelle des Poissons, 

 par M. Agassiz, t. i. + Entwick. des Geburtshelferkrote , p. 10. 



