DEVELOPMENT OP THE CAPE SPECIES OF PERIPATUS. 483 



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 PI. XIV, Part II). The vacuoles of the layer 

 immediately 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 

 (PI. XIV, fig. 23, Part II). 



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

 have already mentioned in Part II of this series {vide figs. 24 b, 

 26 a, on PI. XIV, Part II). At the hind end of the blastopore 

 they become particularly well developed (PI. XIV, fig. 25 «, 

 Part U), 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 difi'ers 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 the primitive streak, in favour 

 of which I may refer to PI. XXXIV, fig. 1, which represents 

 a section through the primitive streak of Stage a, and to 

 PI. XIV, fig. 25 a, Part II, which is a section through the hind 

 end of the blastopore, is the hindermost portion of the blas- 

 topore. 



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 



