78 



EMBRYOLOGY 



vagination of the cesaphageal tnhe (si) takes place, and also the 

 formation of the eight mesenterial septa. The ojsophagus 

 arises in general as an ectodermal invagination (Fig. 33), 

 the bottom of which in later stages breaks through toward 

 the gastral cavity, thereby establishing the inner opening 

 of the cesophagus. The formation of the mesenterial septa 

 is referable to a folding of the entoderm, in which the 

 sustentative lamella also takes part. It seems that in the 



Fig. 32.— Stape in the development of S'jmiiodunn cornllnides (after Kowali.vskt 

 i:t Marion). A and B, cleavage stages; C, embryo vfith permanent entoderm 

 (e)i)and inner mass of detritus (d), in which beginnings of the gastral cavity (h) 

 can be recognized; D, ciliated planula; ec, ectoderm ; en, entoderm. 



Alcyonaria all the eight mesenterial septa always make their 

 appearance at the same time. As regards the origin of 

 the musculature of the septa, especially the longitudinal 

 muscles, authors agree that they arise from epithelio-mus- 

 culai' cells (myoblasts) of the entoderm lamella. 



