DEVELOPMENT OF A CHIM.EROID. 283 



later stages are not as conspicuous. In Fig. 18 is shown a sec- 

 tion of an embryo of about stage C, which is readily comparable 

 with a corresponding stage of Torpedo, as figured, for example, 

 by the Zieglers (Arch. f. mikr. Anat., XXXIX., pi. IV., Fig. 19, 

 VI.) ; an interesting difference is the depth to which the sub- 

 blastoccelic entoderm passes into the germinal yolk at the sides 

 of the gut, a feature which might indeed have been expected in 

 this type of egg. In this connection observe also the fissuring 

 (vacuolar) of the germinal yolk. 



Late Embryos. Embryos of about one eighth inch in length 

 can be readily distinguished from those of sharks. The shape 

 of the head is alone distinctive, for the forebrain vesicle sprouts 

 out like a knob or crest and is a feature characteristic of embryos 



* -*'^ ^v ^i ~"~ivrr**'*-*^,i': : 



: 



- . : . 



FIG. 18. Gastrula. Section through the embryonic region of the stage shown in 

 Fig. 14. 



up to about three fourths inch in length. Other characteristics 

 are the long delicate tail and the short yolk stalk. In still 

 later embryos external gills are developed, in a very shark- 

 like fashion. A spiracle is present in as late a stage as one 

 and a half inches ; it is then a delicate tubular structure and 

 is unprovided with external filaments. Peculiar to the latter 

 is the presence of dilatations, or blood knots, about -^-^ inch 

 in diameter, brilliantly conspicuous by their scarlet color and 

 large size in the living young. They are doubtless places of 

 multiplication (judging from numerous mitoses) of red blood 

 corpuscles. In this stage, as already stated, the embryo is 

 surrounded by a milk-like fluid, which can only be derived 

 from the continued breaking down of the separate yolk masses 

 of an earlier stage (Figs. 5, 13 17) ; that it is nutritive there 

 can be no doubt, and there is every reason for believing, judg- 

 ing from the physiological characters of the \young of other 



