102 :prop. allman on the 



At the pole opposite to that where the gastrula-mouth had 

 been situated, and which is now occupied by the site of the central 

 nervous system, an inbulging of the ectoderm may be seen to 

 take place in the line of the main axis. This gives us the origin 

 of the definitive mouth and of the stomach, which continues to 

 force its way deeper and deeper until it reaches a point a little 

 beyond the middle of the body, and soon opens into the cavity 

 which from a very early period had existed in the interior of the 

 embryo, and which now becomes widened towards the sense-pole, 

 and jforms the foundation of the funnel. Even at this early 

 period the lateral compression of both stomacla and funnel may 

 be seen, the two planes being placed at right angles to one 

 another. 



In the meantime the endoderm had become divided into two 

 halves separated from one another by the plane of the stomach, 

 and each of these again into two halves separated by the funnel- 

 plane. Into each of the four endodermal masses thus differentiated 

 an offset from the funnel extends, gradually advancing towards 

 the mouth-pole, and soon widening into a considerable lumen. 

 In these four sac-like masses of endoderm we have the first ti'aces 

 of the vascular system. 



Between the stomach, funnel, endodermal sacs, and ectoderm 

 the gelatinous tissue had already begun to be secreted as a clear 

 layer, into which Chun had seen the migration of the richly rami- 

 fying cells from the ectoderm and stomach, to which Kowalewsky 

 and Agassiz had already drawn attention. By the continued 

 secretion of the jelly the embryo increases greatly in volume, 

 and the endoderm-sacs become very distinct. 



It is in the stage just described that, according to Chun, most 

 embryos leave the egg, after which the differentiation of the 

 definitive vascular apparatus out of the endoderm-sacs takes place. 

 In JBeroe, however, I have found development proceeding within 

 the egg to a much more advanced stage. 



Chun here makes an important generalization which legiti- 

 mately follows from the course .of the development now traced — 

 namely, that two cavities destined to play different physiological 

 parts in the life of the animal, one being devoted to the diges- 

 tion of the nutriment and the other to its distribution, are 

 derived respectively from two difterent germinal layers. The 

 stomach owes its origin to the ectoderm, being formed by an in- 



