THE DIVISION OF THE YOLK. 59 



* 



In dorsal view the latter stage shows few large yolk masses, and these are 

 distinct from one another, although closely pressed together. The yolk masses, 

 it may be mentioned, were removed separately (in the living condition) without 

 causing their rupture, and it was then seen that the fragmentation of the yolk mass 

 had progressed further than was at first evident, for, lying below and on either 

 side, against the ventral wall of the capsule, were many small masses of yolk (cf. 

 fig. 61, at the right, and fig. 27, somewhat behind the embryo), their contours 

 rounded out for the most part; but a thick, creamy or syrupy fluid in which they lay 

 made it further evident that in some cases the yolk masses had broken down. 

 This fluid, we remark, was observed in specimens of this stage only when the 

 larger masses were separated; but if sea-water was injected (by pipette) between 

 the larger masses as they lay in the open capsule, it would dissolve the underlying 

 creamy yolk and the entire contents of the capsule would become hidden from 

 sight in the resulting milky fluid. 



One might conveniently digress at this point to follow the fate of the yolk 

 masses above mentioned. The blastoderm appropriates only a small portion 

 (which has been estimated as about one-tenth of the volume) of the entire egg. 

 This separate yolk mass is shown slightly shaded in figs. 27 and 61, and the blasto- 

 derm, with its attached embryo, has as yet inclosed only a small portion of it. A 

 similar stage is figured in plate vin, fig. 47, and a somewhat later one in plate viu, 

 fig. 48. In the latter the blastoderm is seen to have almost inclosed the yolk. 

 It completely incloses the yolk and forms a diminutive yolk sac in the embryo 

 shown in plate vin, fig. 49, and a similar condition occurred in the embryo of 

 plate ix, fig. 50. These features are dwelt upon in order to show that the 

 behavior of the blastoderm in appropriating but a portion of the yolk is a normal 

 phenomenon. And I note that the condition shown in fig. 61 has been observed 

 on three occasions by myself, and that similar conditions were recorded by 

 Dr. Wilbur.* In supplementary evidence upon this point we may again refer to 

 the embryo of plate vin, fig. 49, for in this the yolk sac, although of miniature size, 

 is evidently normal, since it exhibits a well-developed vitelline circulation, f 



The yolk-masses other than that appropriated by the blastoderm undergo con 

 tinued subdivision. This is in progress in fig. 61, where the large yolk mass shown 

 in the lower part of the figure is being divided into three smaller ones. We have 

 already referred to the pasty fluid present among the lowermost yolk masses in this 

 stage. In the egg capsule from which the embryo of plate vin, fig. 49, was taken, 

 no extra embryonic yolk masses were found, but the egg capsule contained a fluid so 

 cream-like as to conceal completely the embryo and lead me to infer that the egg 

 was addled, almost causing me to throw away this valuable stage. We can only 

 conclude, therefore, that the creamy fluid was due to the continued breaking down 



*In his early letters Dr. Wilbur referred to these conditions doubtfully; he was then "not sure whether they 

 represented normal appearances." 



fTheyolk sac measured about half an inch in length and a quarter of an inch in breadth; accordingly at this 

 stage of development it represents but about one-tenth the volume of the egg of an Elasmobranch of similar size 

 (e. g., Spinax niger}. 



