C2 CHIIVLEROID FISHES AND THEIR DEVELOPMENT. 



of meroc3-tes in a shorter time.* It thus appears that the early divided merocytes 

 are equivalent morphologically, and probably, therefore, physiologically, to late 

 merocytes in sharks. Furthermore, at the time of fertilization the number of sperms 

 entering the egg of Chimaera appears to be greater than in sharks. The average 

 number reckoned by Riickert in Pristiurus is about i6; in Chimsera it is at least 

 24, judging from the number of entrance funnels in late stages of fertilization. 

 Again, in Chimaera the sperms form and long retain definite paths in the germ which 

 are unknown in other forms; so also are their entrance funnels more conspicuous. 

 The sperm nuclei, furthermore, as well as the segmentation nuclei, are the more 

 special in terms of the entire egg, since they are smaller than in sharks (in diameter 

 about one-half). Also, as evidence of specialization — for these structures have 

 clearly a special physiological value — witness the great number of asters and their 

 read}- mode of appearance, c. g., around vagrant yolk granules of extraordinary 

 size. In point of histological differentiation of the germinal cytoplasm, finally, we 

 observe in Chimaera conditions unparalleled in the shark. We recall here the differ- 

 entiation of typical Schaumplasma, the light areas surrounding the nuclei, and the 

 extensive development of astral ra^^s. 



SEGMENTATION. 



Drawings of the living germ, plate iv, figs. 20-29, give a general idea of the 

 process of segmentation. And in surface view this resembles distincth' the usual 

 conditions in shark. There is the same type of germinal area in which cleavage lines 

 appear, and a marginal zone which apparently circumscribes the area of cleavage. 



In the first of these figures the germinal area appears convex, although some- 

 what flattened above, and is separated from the surrounding germinal yolk by a 

 narroAv fosse. The furrow which appears to traverse it is sharpest and deepest in 

 the middle of the germ and fades awa}- at the margins. The surrounding zone of 

 germinal yolk, however, extends wideh^ over the surface of the egg and lacks a 

 sharph' marked outer boundary line. Its inner boundar^^ /'. e., at the fosse, shows 

 a number of small eminences These, as sections also show, correspond to the 

 eminences formed in the shark blastoderm by peripheral (sperm) meroc3-tes; the}' 

 are more numerous at the corresponding stage and are more regularl}' disposed 

 around the germ. 



In further detail: Sections of the present specimen demonstrate that in spite 

 of the single apparent furrow the present stage represents not the first, but the 

 third cleavage, two cleavage furrows having been retarded ( ? suppressed), for there 

 are found to be present six segmentation nuclei. This condition, it may be 

 remarked, occurs in certain specialized sharks (Torpedo). 



The first division of the segmentation nuclei is accompanied by no trace of a 

 surface furrow. Such a stage is shown in fig. 47 a. The nuclei are here somewhat 

 widely separated from one another and are in the resting stage ; the onh' indication 



*Recent examinations of the sections of the fertilization stage which yielded fig. 46 a (Chimaera Emb. 12, in my 

 cabinet) show that no less than 88 (sperm) merocytes are present. Thus in Chimaera as many merocytes are present 

 in a presegmentation stage as in Torpedo (Riickert) in a stage of fourth cleavage. 



