88 EMBRYOLOGY 



trie space of the axis into a dorsal and ventral canal, there is 

 found a calcareous axis (ectodermal according to v. Koch's 

 conjecture), surrounded by an axial epithelium, and two 

 lateral canals lying at the sides of the former, which, as 

 nutritive or sap canals, belong to the gastrovascular system. 



From the embryology it appears that the older authors 

 have employed the expressions " ventral " and " dorsal " for 

 the Pennatula colony in the opposite sense to that which 

 is admissible according to the orientation of the axial polyp 

 (Jungersen). 



Zoantharia. — In the majority of cases fertilization and 

 cleavage take place inside the mesenterial septa, and the 

 further development, as far as the complete formation of 

 the planula, in the gastral cavity of the parent. In this 

 stage the lai^vse are cast out through the moath-opening. 

 On the other hand, Cerianthus membranaceus and Actinia 

 parasitica (Adamsia Rondeletii), according to Kowalevsky, 

 eject the spawn in an unsegmented condition. 



Considerable uncertainty still prevails regarding the 

 earliest developmental processes, the knowledge of which 

 we owe chiefly to Kowalevsky (No. 10), Jourdan (No. 80), 

 and H. V. Wilson (No. 99). In many cases cleavage and 

 the differentiation of the entoderm seem to take place in 

 connection with the formation of a solid morula, therefore 

 in a manner similar to that which has been described for 

 the Alcyonaria. At least there is in support of this Kowa- 

 levsky's observation on Actinia parasitica (Adamsia Ronde- 

 letii), which is described in the following manner: "Cleav- 

 age is regular, but as the result of it there arises not a 

 blastodermic vesicle, but only an aggregation of cells, which 

 becomes covered with cilia, and swims about as a larva; 

 subsequently a small depression is formed at one spot. The 

 opacity of the eggs made a further pursuit of the develop- 

 ment impossible." The author is convinced that the ento- 

 derm in this case is not formed by invagination, but by a 

 splitting off from the blastoderm, as in the Corallia. In 

 sections through ciliated larvae of Astroea Kowalevsky found 

 the two layers, ectoderm and entoderm, composed of cylin- 

 drical cells, and an inner contained mass, which had obvi- 



