DEVELOPMENT OF A CHIM/EROID. 2/3 



lamellae, and around their delicate tips the thin walls of the 

 shell weather away so that, by the time of hatching, the inter- 

 locking 1 lamellae can disengage and thus permit the young 

 fish to push its way through the folds. The exit-slit of the egg- 

 case is thus a line of rupture. By somewhat similar lamellae 

 framing a bilateral series of fenestrae ventilation within the 

 case is obtained and perfected in later stages. The fenestrae 

 (as in eggs of certain elasmobranchs) occur in the hinder 

 end of the case, and in latest stages water flows through them, 

 a current being created by the constantly undulating tail of 

 the embryo. The latter has at all times a highly specialized 

 relation to the egg-case, as is indicated in Figs. 2-7. As soon 

 as the embryo can be determined its head points toward the 

 opening-end of the case, the keeled side of the case being 

 dorsal. It s is evident that the narrow end becomes the neatly 

 fitted sheath for the elongated tail, and that the widened 

 end fits accurately the greatly enlarged head and trunk. The 

 young hatches as soon as the straight egg-case is filled ; thus 

 there is no coiling and wrapping up of the later embryos, as is 

 found, for example, in the case of many sharks. 



The Duration of Development. After the egg is deposited (water 

 temperature 50 60 F.) fertilization stages continue for about 

 twenty-six hours. If, therefore, we assume that the formation of 

 the egg-case began with fertilization the total duration of this 

 process is probably not less than three days, and not longer than 

 five. 



Segmentation (of nuclei), beginning about twenty-six hours 

 after the egg is deposited, lasts from three to four days, each 

 earlier cleavage taking about two hours. 



A well-marked blastula is I ip hours old (Fig. 2). 



Gastrulation dates from about the eighth day. On the tenth 

 day the embryo resembles Balfour's stage C (Pristiunis) (Fig. 3) ; 

 on the seventeenth, D ; on the nineteenth, E; and on the twenty- 

 second, F (Fig. 4). 



Of later embryos one resembling stage / is thirty-one days old 

 (Fig. 5). I have no accurate data of older stages. Assuming, 



1 A row is made up of two sets of interlocking elements, one passing to the right, 

 one to the left, like fingers of interlocked hands. 



