484 ADAM SEDGWICK. 



like connective-tissue cells ; a feature which is not at all 

 exaggerated in the figures. 



The nuclei of the lips of the blastopore, which have already- 

 been described as intermediate between ectodermal and endo- 

 dermal nuclei, constantly present the karyokinetic figures cha- 

 racteristic of dividing nuclei (PI. XXXIV, fig. 3, d. n., and 

 PI. XIV, fig. 26 h, Part II). 



During Stages b and c, the endoderm, though diminishing 

 somewhat in thickness, retains all the characteristics just 

 described. 



During Stage b the blastopore, which has grown considerably 

 in length, and markedly dumbbell-shaped in surface views closes 

 in its middle portion (PI. XXXII, figs. 23—25, Part I). This 

 is effected simply by the approximation and fusion of its lips 

 (PL XXXIV, fig. 5 6). The connection, which has hitherto 

 existed between the latter by the anastomosing strands already 

 mentioned, now becomes closer and they completely unite with 

 one another. The result of their union is that the intermediate 

 nuclei come to lie inside and form a definite part of the endo- 

 derm, viz. the ventral endoderm in the median line between 

 the mouth and the anus (PI. XXXIV, fig. 6 c). By their fate 

 then these intermediate nuclei are endodermal, and so indeed 

 I think we must regard them, unless indeed we are willing to 

 take the view that the median ventral endoderm of the ali- 

 mentary canal of Peripatus is ectodermal in origin. Up to the 

 close of Stage c the endoderm has been in close contact and 

 continuous with the ectoderm, excepting where the mesoblastic 

 somites, which appear in Stage b, intervene. In Stage d, 

 however, a remarkable change takes place, the endoderm 

 separates ventrally and dorso-laterally from the ectoderm, and 

 there is now a direct connection between the two only along 

 the dorsal middle line (PL XXXIV, fig. 9). This is soon lost 

 in Stage e, and henceforth the endoderm layer is only connected 

 with the ectoderm through the walls and cells of the meso- 

 blastic somites, and at the mouth and anus. 



This completes all I have to say about the endoderm till the 

 close of Stage e, when it consists of a layer of vacuolated 



