66 THE CHANGES FROM STAGE A TO F. 



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 (Plate VI, fig, 3, d. n., and 

 PI. V, fig. 26 b). 



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 (Plate II, figs. 23 — 25). This is effected 

 simply by the approximation and fusion of its lips (Plate VI, 

 fig. 5 b). 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 endoderm, viz. 

 the ventral endoderm in the median line between the mouth 

 and the anus (Plate VI, fig. 6 c). By their fate then these 

 intermediate nuclei are endodermal, and so indeed I think we 

 must regard th6m, unless we are willing to take the view that 

 the median ventral endoderm of the alimentary canal of Peri- 

 patus 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 n, however, a remark- 

 able 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 

 (Plate VI, fig. 9). This is soon lost in Stage e and hence- 

 forth the endoderm layer is only connected with the ectoderm 

 through the walls and cells of the mesoblastic 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 



