GASTRULATION AND EARLIEST DEVELOPMENT 171' 



size. We may distinguish a central group of large, yolk-laden, 

 inactive cells and a peripheral ring of smaller and more 

 active entoderm- cells which, by their proliferation and 

 moving inwards, contribute especially to the invagination 

 and the formation of ihe archenteric cavity. The wall 

 of this cavity in Amphioxus consists partly of the 

 larger entoderm-cells and in Amphibians at the corres- 

 ponding place it is thickened very much by the presence of 

 the mass of yolk-cells which as a voluminous plug project 

 into the archenteron, often filling it up for the greater part 

 and leaving only a ring-shaped lumen. The ring of smaller 

 entoderm-cells is best developed near the anterior border 

 of the entoderm-area and as a consequence the archenteric 

 cavity at the close of gastrulation is best developed under 

 the dorsal blastopore border. This causes the plug of 

 yolk-cells to lie not in the centre of the archenteric wall, 

 opposite the blastopore, but more to the ventral side. The 

 dorsal wall then is formed especially by the smaller ento- 

 derm-cells, which contain much less yolk and from which 

 afterwards the notochord and the mesoderm are derived. 

 In Amphioxus the difference in size of the dorsal and the 

 ventral entoderm-cells is insignificant, in Amphibians it is 

 already greater and it becomes very considerable in the 

 yolk-laden eggs of Selachians and Amniotes. Here as a 

 matter of fact the cells of the dorsal archenteron wall by 

 their size and appearance stand much nearer to the ecto- 

 derm-cells than to the often enormous yolk-laden ventral 

 entoderm-cells, and this will easily give rise to the impression 

 that an invagination of ectoderm-cells has occurred round 

 the dorsal border of the blastopore. These cells would 

 have formed the dorsal archenteron wall from which the 

 notochord and the mesoderm will be derived afterwards. 

 This view has been advocated indeed as early as 1879 

 by Scott and OSBORNE (1879) in their study on the 

 development of the newt, and in 1882 and '83 0. HERTWlG 

 came to similar conclusions. The latter identifies the pig- 

 mented animal half of the frogs egg with the ectoblast, 

 the unpigmented vegetative half with the entoblast. During 

 gastrulation the "animal" cells at the dorsal border invagi- 

 nate in such a way that the median band of the archen- 

 teron roof, from which the notochord originates, is formed 

 by them (p. 262-264). The mesoblast-bands are also derived 

 from the animal cells (p. 263). 



