52 



S. H ATTA. THE KATE OF THE PERISTOMAL 



reduced and finally closed, is founded by further differentiation of one of the 

 two layers brought about by the overgrowth. While the outer layer of the 

 two, the ectoderm, thickens itself to the medullary plate, the inner layer, 

 the entoderm, roofing over the archenteron, differentiates into the chorda 

 and into the mesoderm. In the cephalic region, where the archenteric 

 roof is broad enough to be folded, the mesoderm is formed by the folding 

 of the layer, but, as the roof is narrower according to the narrowed lumen 

 of the archenteron towards the hind extremity of the embryo, the lumen of 

 the mesodermal pocket is gradually obliterated in this direction. In the 

 posterior largest section of the body the mesodermal folding of the archente- 

 ric roof is performed by cell-proliferation taking place on either side of 

 the rudiment of the chorda and is indicated merely by the position of the 

 nuclear spindle of cell division which shows that the produced cells are 

 being put into a layer folding, on cross-sections, like that done by the true 

 folding. The mesectoderm, whether it is formed by folding or by proli- 

 feration, is wedged in between the enteroderm and the entoderm. The 

 median part of the latter is thickened to the medullary plate, wherever 

 this is pronounced. The hindmost portion of the peristomal mesoderm is, 

 just before the complete closure of the blastopore, wedged in between the 

 cnteioderm, the yolk-cells, and the ectoderm forming the ventral blasto- 

 poric lip (Photogram la.) 



Before the blastopore is completely closed, the hindermost portion of 

 the peristomal mesoderm undergoes peculiar changes. The outer layer of 

 the ventral blastoporic lip, which can now distinctly be designated as the 

 ectoderm, sinks in and gives rise to a short, median, longitudinal slit, the 

 proctodaeum. Now, the circular lip of the blastopore is divided by the 

 dorsal notch and by the ventral proctodaeal indentation into the right and 

 left wings. Then follows the stage, in which the wings are turned up, so 

 as to be raised almost into horizontal position instead of the vertical which 

 they had until then assumed. By this change the dorsal. median notch on 

 the lip is reduced so as to form the anterior corner of a fine slit. The 

 hindermost portion of the peristomal mesoderm, which is put inquestion, . 



