90 ALIMENTARY CANAL. 



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 

 (PI. IX. 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 

 beo^ins to be folded off behind. After the foldino^ has once com- 

 menced 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 

 end of the body, they meet there, their walls coalesce, and a 

 direct communication from the neural to the alimentary canal 



in protoplasm, and the meroLlastic ovnm as a body constituted of the same es- 

 sential parts as a holoblastic ovum, though divided into regions which differ in 

 the proportion of protoplasm they contain. J do not proi:)Ose 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 extending 

 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 proto- 

 plasm ; 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, PL ix. 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 produced 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. 



