DEVELOPMENT OF ANTEDON. 



155 



Antedon* first made us familiar with the conception of the ante- 

 rior coelom and so paved the way for the modern views, largely 

 due to him and MacBride, on the segmentation of the coelom 

 into three chambers on each side of the body (p. 149). 



B 



_-*r"v VT 



mes-. 



A> 



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^S?BB3j3 

 $T $#. * 



M<^&iM 



-mot 



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Fia. 111. 4 longitudinal vertical section of gastrula of Antedon at the end of the second 

 day showing the formation of mesenchyme. bl blastopore (after Bury). B longitudinal 

 section of a later stage showing the division of the archenteron. mes mesenchyme ; 

 ent first-formed enterocoel which gives rise to the right and left posterior coeloms (after 

 Barrois). 



After the closure of the blastopore the archenteron which is 

 placed at the hind end of the embryo divides by a constriction 

 into an anterior and a posterior portion (Fig. Ill B). The 

 posterior portion is the first 

 enterocoel vesicle ; it lies 

 close to the hind end of the 

 embryo and soon divides into 

 a right and left part, which 

 constitute the right and left 

 posterior coeloms respec- 

 tively. The anterior portion 

 or vesicle into which the 

 archenteron has divided de- 

 velops two outgrowths, a 

 dorsal and a ventral (Fig. 

 112, 5, 7), which give origin 

 to the intestine ; and itself 



becomes constricted into two portions, a ventral one which 

 is the rudiment of the hydrocoel, and an anterior one which 

 is the anterior coelom (Fig. 112). These become separated, in 



* Phil. Trans., 179, 1888. 



ia. 112. Posterior end of an embryo of Ante- 

 don of sixty hours, from the right side. 1 the 

 outline of the right posterior coelom ; 2 rudi- 

 ment of anterior coelom ; 3 rudiment of 

 hydrocoel ; 4 mesenchyme ; 5 ventral, 7 

 dorsal part of enteron, which clasps 6 the still 

 persistent connecting portion between the 

 incipient right and left posterior coeloms (from 

 Lang, after Seeliger). 



