92 



I do not propose here to bring into review the many theories 

 which have been proposed with regard to the above processes of 

 Teleostean development. As has already been said, most have 

 arisen from the failure to recognise the true importance of the 

 entoderm, the existence of which was either ignored or wTongly 

 interpreted. It cannot be said either that the entoderm does not 

 take part in the formation of the embrj^o, since it ;v^elds the ento- 

 derm of the embryo and participates to such a large extent in the 

 formation of the notochord and the mesoderm. 



The Peripheral Protoplasm of the Yolk. — The next point 

 to which I Tidsh to direct attention is the fate of the peripheral 

 protoplasm of the yolk. Notwithstanding the fact that in the 

 sections we have just been considering of third day embryos, the 

 embryonic area has extended to about a third of the superficial 

 area of the egg, the margin has still the same structure as before. 

 The outer laj^er of ectoderm is continued into the protoplasmic 

 sheet which en^vraps the yolk. The sjnicj'tial entoderm runs 

 similarly into the peripheral protoplasm, and just before the two 

 layers join they are connected by protoplasmic bands. The whole 

 margin presents the same structure. The figs, ^a-d will show 

 that during the subsequent stages when the yolk is being surrounded 

 by the blastoporal grow^th the margin has still the same structure. 

 In the case of figs. 5a and 5c [see fig. 4, page 84) the margin occupies 

 the equatorial region of the egg, and in fig. 5^ {see fig. 5, page 84) 

 the Hps are being approximated at the vegetal pole. In each case 

 it will be seen that the margin is proHferating ectoderm, entoderm 

 and mesoderm, and that the investing cytoplasm of the yolk is 

 progressively divided into an outer and an inner layer connected 

 by strands of intervening protoplasm. It is clear also that the 

 outer layer is continuous all the time with the outer ectodermal 

 layer, and the inner with the entoderm. There is no difference 

 in this respect at any stage in any part of the margin. In other 

 wordS; the peripheral protoplasm of the yolk is not lost ; it is pro- 

 gressively excavated, and the resulting sheets of protoplasm form 

 the bases of or are incorporated with the outer layer of ectoderm 

 and the S3Ticytial entoderm. It has already been shown that the 

 polar mass of protoplasm is resolved during segmentation into an 

 outer ectoderm and an inner entoderm. This is clearly also the 

 case with regard to the peripheral protoplasm of the yolk. 



