78 



EMBRYOLÜGY 



vagination of the oesophageal tube (si) takes place, and also the 

 formation of the eight mesenterial septa. The oesophagus 

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

 the bottom of which in later stages breaks through toward 

 the gastral cavity, thei'eby establishing the inner opening 

 of the oesophagus. 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.— Stage in the development of S'jmpodium coralloides (after Kowalevskt 

 HT Marion). A and B, cleavage stages; C, embryo with permanent entoderm 

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

 can be recognized; D, ciliated planula; ec, ectoderm ; c?i, 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- 

 cular cells (myoblasts) of the entoderm lamella. 



