6 2 CHIM.EROID FISHES AND THEIR DEVELOPMENT. 



is opposed, on the other hand, by the conditions in the egg of Cestracion, where 

 the peripheral furrows, similar in general regards, are known to be continuous with 

 those of true cleavage. The question, therefore, can not be answered finally until 

 evidence is forthcoming to distinguish the kinds of nuclei present in the extra- 

 embryonic yolk masses. Meanwhile, judging at least from the behavior of the 

 nuclei in the circumgerminal ring, I think it is not at all improbable (cf. Gastru- 

 lation) that in these masses both sperm and segmentation nuclei are present. 



Returning again to the development proceeding at the animal pole of the egg: 

 We recall that in the sections figs. 53-56 there was shown a stage of late seg- 

 mentation, or an early blastula, such, for example, as pictured in plate iv, fig. 27. 

 In a slightly later stage (plate iv, fig. 28) an increased number of blastomeres are 

 present, and there is still an indefinite condition in the periphery of the germ, 

 blastomeres being continued irregularly over the ring-like circumgerminal zone. 

 On the other hand, in plate iv, fig. 29, a stage is figured earlier than the preceding, 



Fig. 62. Section of blastula. sc, Segmentation cavity. 



but showing a well-marked line of demarcation between the blastomeres and the cir- 

 cumgerminal zone. It seems evident, accordingly, from this and similar instances, 

 that considerable variation occurs as to the time at which the marginal relations of 

 the germ are established. Thus in the stage first referred to (figs. 53-56) the 

 circumgerminal zone was traversed by radial fissures and invaded by nests of cells; 

 in a similar stage (plate iv, fig. 29, sectioned in fig. 62) the same region is solid and 

 yolk-filled, forming a compact border to the germ. 



In contrasting these two stages one observes that, while they can differ little in 

 point of age, judging from the number of blastomeres in the cross section of the 

 m iddle of the germ, they yet have marked differences in their relation to the yolk ; the 

 former has around it and under it "fine yolk" (Riickert); the latter has its fine yolk 

 contracted into a thick mass lying immediately below the germ, a condition which 

 may be the immediate cause of the failure of marginal blastomeres to express 

 themselves in a peripheral direction. We observe that in fig. 62 the fine yolk is 

 pervaded with vacuoles which, from their shape and relations, are evidently equiv- 

 alent to inter-blastomeral spaces, a conclusion which is supported both by the 

 nucleated character of the masses of fine yolk thus outlined and by the continua- 

 tion of the inter-blastomeral spaces with the distal ends of the vacuoles. The fine 

 yolk, in short, is already coming to be formed into blastomeres, and it is interesting 

 to note that a blastomere, which is found on the boundary line between the fine 



