THE NUCLEI OF THE ENDODERM. 65 



The Endoderm. 



The endoderm during Stage a and earlier stages consists 

 simply of the inner portion of the vacuolated protoplasmic 

 wall of the embryo. Its vacuoles are somewhat larger and its 

 nuclei fewer and more irregular in shape than those in the outer 

 or ectodermal portion. But the two are perfectly continuous 

 {vide figures on Plate V). The vacuoles of the layer imme- 

 diately within the ectodermal nuclei are larger than those in the 

 innermost layer, i. e. in the layer next the gut cavity. 



Processes from the endoderm cells project into the enteron 

 and anastomose with each other. This is a well-marked feature 

 in embryos rather younger than Stage a, and indicates the 

 origin of the gut as a vacuole or a concresence of vacuoles 

 (Plate V, fig. 23). 



These processes persist until Stage b at the blastopore, as I 

 have already mentioned in Chapter II {vide figs. 24 b, 26 a, 

 on Plate V). At the hind end of the blastopore they become 

 particularly well developed (Plate V, fig. 25 a), so much so 

 that it is impossible to say in which section the blastopore ends 

 and the primitive streak begins. In other words, the blastopore 

 passes quite gradually into the primitive streak. Or again, 

 to put it another way, the primitive streak only differs from 

 the hind end of the blastopore in the fact that the anastomosing 

 protoplasmic strands, which everywhere traverse the blastopore, 

 contain nuclei in the former case, but not in the latter. On 

 this view, in favour of which I may refer to Plate VI. fig. 1, 

 which represents a section through the primitive streak of 

 Stage A, and to Plate V, fig. 25 a, which is a section through 

 the hind end of the blastopore, the primitive streak is the 

 hindermost portion of the blastopore. 



The nuclei of the endoderm are large, and particularly 

 remarkable for the irregularity of their shape. They do not, 

 excepting those near the lips of the blastopore, ever present 

 the karyokinetic figures characteristic of dividing nuclei ; they 

 appear to divide directly. Some of them are much branched 



