78 CHANGES IN THE OVUM 



the two lateral masses, the laminae dorsales. After the formation of yolk- 

 cells has proceeded to a certain extent in the cortical part of the ovum, 

 a similar process commences in the central portion. In this part of the 

 ovum, however, there are originally no germinal cells ; and it would seem, 

 according to Vogt,* that at this period clear transparent cells, exactly 

 similar to the germinal cells, are developed here ; and that then an in- 

 vesting membrane forms around each of them, and thus produces so many 

 yolk-cells in the same manner as those of the cortical part are formed. 

 The cells of the central part of the ovum differ, however, from the cortical 

 ones, in being larger, of an irregular form, and possessing stearine, instead 

 of granular matter, as their contents. During the early period of the cell- 

 formation in the central part of the ovum, gradual transitions in form and 

 other characters, from the cortical to the central cells, may be perceived as 

 the ovum is examined from without inwards. But at a later period the 

 cortical cells become more and more distinctly separated from the central 

 ones, and eventually constitute a perfectly distinct layer, which can be 

 completely separated from the central portion of the ovum, without injury 

 to either. On comparing the description furnished by Reichertf of the 

 composition of the frog's ovum subsequent to impregnation, with Vogt's 

 account as given above, it will be observed that, at least with regard to the 

 variety in size of the cells of the yolk, a close similarity exists between 

 them. But concerning the nature of these two kinds of cells or corpus- 

 cules, the changes which subsequently ensue in them, and the relative share 

 possessed by each in the formation of the embryo, considerable discrepancy 

 is observed in their opinions. By Reichert it was assumed that the large 

 central cells, which at first are unprovided with nuclei, subsequently became 

 nucleated, that young cells are then developed within them, and that, in 

 proportion as they approach the periphery of the yolk so do the young 

 corpuscules within increase in size ; and eventually the membranous wall 

 of the parent-cell disappears, while the small liberated cells constitute the 

 formative mass out of which the several parts of the embryo are deve- 

 loped. And this, he considers, goes on until the whole of the large yolk- 

 cells are resolved into broods of smaller cells ; and the nearer these larger 

 cells are to the centre of the yolk, the later are they in the production of 

 young cells in their interior. The whole of this view, however, is contra- 

 dicted by Vogt, | who observes that, at least in the ovum of the toad, no 

 trace of young cells contained within parent-cells can be detected at the 

 centre or any other part of the substance of the yolk; and he considers that 

 the young cell which Reichert figures within a larger one is simply the 

 germinal cell, around which the larger yolk-cell has formed, in the manner 

 already described. In Vogt's opinion, as already expressed, the peripheral 



* Entwick. des Geburtshelferkrb'te, p. 11. t Mullet's Physiology, p. 1512. 

 J Entwick. des Geburtshelferkrote, pp. 34 40. 

 See fig. 170, p. 1513, Mullet's Physiology. 



