44 



CHIM^JROID FISHES AND THEIR DEVELOPMENT. 



15 mm.) the conditions of the tunic have changed. It has become thinner (fig. 32) 

 and has modified its structures considerably. In place of the differentiated inmost, 

 middle, and outmost layers, an outer layer is alone conspicuous and even here the 

 nuclei have not the crowded character of the earlier stage ; the inmost and middle 

 layers have merged, forming a somewhat indefinite layer, poor in nuclei. In some 

 cases markings in the (partial) syncytium indicate that the tunic is in places but one 

 cell thick. Indirect divisions sometimes occur. The entire structure of the tunic 

 suggests that closer physiological relations exist between the vascular supply, on 

 the one hand plasma spaces, as in c, having now extensively drawn together into 



d 



cy 



Sf 



Fig. 34. Section of germinal region of egg of Chimeera colliei during stage of early fertilization. cy. Coarse 

 yolk ; li, dendritic lines marking paths of sperms ; fy, fine yolk ; gp, germinal area extending as a 

 plug-shaped mass deep into the egg ; n, nuclei, sperm, and egg fusing. Under the points marked with 

 asterisks (*) vacuoles occur which separate masses of fine germinal yolk and are usually found to contain 

 sperm nuclei. 



capillaries and the egg on the other. This physiological process is doubtless aided 

 by the thinning of the membrana limitans, bm, and of the zona radiata. And as 

 an indication that the nutriment is being passed rapidly into the yolk we observe 

 that even close to the surface of the egg large yolk masses are appearing. 



In a final stage of the ovarian egg the tunic is still further diminished in thickness 

 (figs. 33, 33 A). It is reduced, in fact, to a single-celled layer, of which the nuclei 

 are usually disposed nearer the outer wall in the same niveau. Sometimes, however, 

 they are closely apposed to the inner wall, but in all cases they are of the same 

 general character (fig. 33 B), i. e., spherical nuclei, with sharply marked mem- 

 brane, showing but faint traces of chromatin meshwork, but with one or two con- 



