306 DEVELOPMENT OF ELASMOBRANCH FISHES. 



The history of the alimentary canal during the remainder of 

 this period may be told briefly. 



The folding off and closing of the alimentary canal in the 

 anterior part of the body proceeds rapidly, and by stage D not only 

 is a considerable tract of alimentary canal formed, but a great 

 part of the head is completely folded- off from the yolk (PL 10, 

 fig. 3 a). By stage F a still greater part is folded off. The 

 posterior part of the alimentary canal retains for a long period 

 its primitive condition. It is not until stage F that it begins to 

 be folded off behind. After the folding has once commenced it 

 proceeds with great rapidity, and before stage G the hinder part 

 of the alimentary canal becomes completely closed in. 



The folding in of the gut is produced by two lateral folds, 

 and the gut is not closed posteriorly. 



It may be remembered that the neural canal also remained 

 open behind. Thus both the neural and alimentary canals are 

 open behind ; and, since both of them extend to the posterior 



constituted of the same essential parts as a holoblastic ovum, though divided into 

 regions which differ in the proportion of protoplasm they contain. I do not propose 

 to repeat the positive arguments used by me in favour of this view, but content 

 myself with alluding to the protoplasmic network found by Schultz and myself ex- 

 tending through the whole yolk, and to the similar network described by Bambeke 

 as being present in the eggs of Osseous Fish after deposition but before impregnation. 

 The existence of these networks is to me a conclusive proof of the correctness of my 

 views. I admit that in Teleostei the 'parablast' contains more protoplasm than the 

 homologous material in the Elasmobranch ovum, while it is probable that after 

 impregnation the true yolk of Teleostei contains little or no protoplasm; but these 

 facts do not appear to me to militate against my views. 



I agree with Prof. Bambeke in regarding the cells derived from the sub-germinal 

 matter as homologous with the so-called yolk-cells of the Amphibian embryo. 



I have recently, in some of the later stages of development, met with very 

 peculiar nuclei of the yolk immediately beneath the blastoderm at some little 

 distance from the embryo, PI. 10, fig. 8. They were situated not in finely sub- 

 germinal matter, but amongst large yolk-spherules. They were very large, and 

 presented still more peculiar forms than those already described by me, being pro- 

 duced into numerous long filiform processes. The processes from the various nuclei 

 were sometimes united together, forming a regular network of nuclei quite unlike 

 anything that I have previously seen described. 



The sub-germinal matter, in which the nuclei are usually formed, becomes during 

 the later stages of development far richer in protoplasm than during the earlier. It 

 continually arises at fresh points, and often attains to considerable dimensions, no 

 doubt by feeding on yolk-spherules. Its development appears to be determined by 

 the necessities of growth in the blastoderm or embryo. 



