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CASWELL GRAVE 



those cells which form the archenteron (mesoderm and endoderm), 

 and a zone situated at and around the animal pole of the egg 

 containing material for the ectoderm cells. 



Morgan and Spoon'er ('09) have since clearly shown that these 

 visible formative stuffs are not the determiners of the mor- 



Text fig. 2 Schematic drawings to show the place and method of origin in 

 development of certain parts of the coelom and the hydrocoele in the late gastrula 

 stage of various echinoderms. 



A and D are representative of the manner of differentiation of these meso- 

 dermal structures typical for then umerous species of Ophiurids, Echinids and 

 Asterids which produce eggs of small size with comparatively small supply of 

 yolk material. 



The remaining figures represent what has been found to be the order of events 

 in the development of certain species producing large yolky eggs: B, Ophiura 

 brevispina; C, Antedon; E, Solaster; F, a holothurian. 



In B, C, and E the blastopore closes soon after gastrulation, and the archen- 

 teron looses its connection with the wall of the gastrula. The dotted lines show 

 the position formerly occupied by the archenteron and blastopore. 



a, archenteron; 1, left anterior enterocoele; 2, right anterior enterocoele; 

 3, left posterior enterocoele; 4> right posterior enterocoele; 5, hydrocoele. 



