60 CHIM;EROID FISHES AND THEIR DEVELOPMENT. 



of the yolk masses of the earlier stage.* That this fluid was nutritive to the 

 embryo was also evident, since the external gills were dilated at various points 

 with brilliantly colored blood knots, and in these, as I later found, numerous 

 erythrocytes were undergoing division. And this condition in the gill filaments 

 is the more clearly correlated with the presence of the milky fluid, since in similar 

 egg capsules (sharks and rays), where this milky fluid mass is lacking, blood knots 

 on the external gills are also absent. One infers, moreover, that the milky fluid, 

 which from its included yolk is highly nutritive, may also be passed as food into 

 the mouth of the embryo and assimilated in the gut. But to this I will refer at a 

 later point. 



The entire process of the fragmentation of the egg of Chimaera, it will be seen, 

 is worthy of especial comment. Unlike the eggs of other vertebrates, and unlike, 

 indeed, those of invertebrates, unless we include a somewhat generic resemblance 

 in certain mollusks (e. g., Neritina, Blochmann, 1887) and in certain digenetic 

 trematodes, the present egg follows in its development two distinct paths, i. e., a 

 small portion of the egg develops in the direction of producing the embryo with 

 its complete though diminutive yolk sac; the remaining portion, about nine-tenths 

 of the bulk of the egg, proceeds to undergo a process of repeated fragmentation to 

 the end that it may be appropriated by the embryo secondarily. 



To account on phyletic grounds for this extraordinary and "unnatural" plan of 

 development, one must, I believe, start with the premise that the fragmentation 

 of the egg is a process comparable with total cleavage. This premise we may 

 accept on the following evidence: 



(1) The fragmentation, like cleavage, is progressive. 



(2) Although the cleavage lines have never been followed conclusively from 

 the rim of the blastoderm into the deep fissures which initiate the fragmentation, 

 they have at least been observed in late stages of segmentation to pass out over the 

 circumgerminal zone in the direction of the peripheral fissures (cf. in this regard 

 the evidence of Heterodontus).f 



(3) The yolk masses give evidence of being nucleated. There is in the first 

 place evidence that the nuclei travel peripherad. In the stages of plate iv, figs. 

 25-27, nuclei are found to have occupied the circumgerminal zone, i. e., they have 

 traveled outward a distance equal to about three-quarters of the diameter of the 

 blastoderm. In an early gastrula, furthermore (plate v, fig. 31), and in section, 

 fig. 63, they have proceeded outward a distance equal to twice the diameter of the 

 blastoderm. Now, on the evidence of progressive centrifugal movement of the 



*The reader may reasonably query at this point how it happens that the creamy nutritive material is not washed 

 out through the openings of the capsule during the respiration of the young. This result has, I take it, been avoided 

 in the course of the evolution of this process in two ways : (i) By retarding the appearance and growth of the capsular 

 openings until the nutritive material is partly consumed ; (n) by the great density of the creamy fluid, for if the 

 nutritive fluid be heavy (and experiments with the living eggs have convinced me that this is the fact), a moderate 

 current of sea-water could be passed over it without causing it to be washed away. 



tAnnot. Zool. Jap., 1901, vol. iv, pt. I, pp. 1-7. 



