5o 



S. H ATTA. TUE FATE OK THE PERISTOMAL 



obvious that the prominence of the ridge depends partly upon the depth to 

 which the macromeric surface of egg embraced by the two arms of the 

 ridge sinks in, but mainly on the extent to which the micromeric layer 

 folded and grows inwards. At the middle part of the arc where the ridge 

 is most prominent, the ingrowth of the micromeric layer is greatest in 

 extent and becomes less as we proceed towards the extremities of both 

 the arms of the arc, until at last the layer does no longer grow inwards. 



By stages the ridge extends ventrally, so that its extremities on both 

 sides are brought nearer and finally meet with each other at the middle 

 point of the ventral blastoporic lip, while the mocromeric surface of egg is 

 depressed and is surrounded by the ridge all around. At the same time 

 the ridged lateral lips shift so as to approach one another and the median 

 line, and as a consequence the arc is converted into an equilateral triangle 

 whose apical angle represents the dorsal blastoporic lip. This angle is not 

 acute, but takes the form of a rounded notch (Hatta, fig. 9c). 



The notch passes, on one hand, gradually into the dorsal groove 1 * 

 running lengthwise on the dorsal median line and leads, on the other, into 

 the archenteron which is at first very short, but becomes longer by 

 degrees. 



The dorsal and the lateral lips of the blastopore are not stationary, 

 but grow posteriorly, as the following facts make evident. 



In the first place, as time goes on the archenteric pocket is deepened. 

 In consequence the roof of the cavity is added to in the direction of its 

 long axis. The outer layer which covers the archenteric roof, the 

 ectoderm, undergoes, of course, the parallel anteroposterior extension. 

 In the second place, the blastopore is reduced by stages, i.e. the lateral 

 lips of its shortened. (Hatta, 07, figs. 8c, gc, I oc). As the ventral blasto- 

 poric lip is stationary so long as both the ridged arms of the lateral lips are 

 prolonged to meet, on the median point of the ventral lip, it is obvious 

 that the reduction of the blastopore is carried on from before backwards. 



1) This groove is identical with the "Rückenrinne" of German authors, or the primtive groove 

 in higher Craniota. 



