76 



EMBRYOLOGY 



mesenterial septa. It is here also that the eg-gs in most 

 cases ai'e fertih'zed, and frequently undergo the first stages 

 of development, viz., cleavage and the formation of a 

 spheroidal embryo consisting of two germ-layers. The 

 embryo is afterwards set free in the gastral cavity of the 

 parent, from which it is ejected thi-ough the mouth-opening, 

 usually in the stage of a ciliated planula. While thus many 

 Alcyonaria are viviparous, cases have also been observed in 

 which the eggs, either unfertilized or immediately after 

 fertilization has taken place, are extruded through the mouth- 

 opening of the parent, either singly or united into Inrge 

 masses by means of a slimy siibstance (Alcyonium, Renilla, 

 Clavularia crassa). 



The eai-ly development of the Alcyonaria has become 

 known chiefly through Lacaze-Duthiers (No. 88, Corallium), 

 KowALEVSKY (No. 10, Alcyonium, Gorgonia), v. Koch (No. 

 86, Gorgonia), E. B. Wilson (No. 98, Renilla), and KowA- 

 LEVSKY ET Maiugn (No. 87, Clavularia, Sympodium). 



The ripe egg of the Alcyonaria is usually rather rich in 

 granules of food-yolk, which, mixed with oil drops, is accu- 

 mulated especially in the inner parts, so that in certain cases 

 there is a sharp separation of a finely granular ectoplasm 

 from an endoplasm rich in food-j'olk. Cleavage has been quite 

 variously described for the forms so far observed ; in fact, in 

 Renilla it exhibits remarkable individual variations. In 

 general it follows the total and equal type, and finally leads 

 to the development of a solid so-called morula-stage, consist- 

 ing of cells more or less uniform in size and exhibiting even at 

 an early stage a difference between the more finely granular 

 cells of the superficial layer and the coarsely granular 

 ones of the inner mass. An interesting modification of the 

 cleavage pi-ocess is met with frequently in Renilla, and con- 

 stantly in Clavularia crassa. Here a multiplication of the 

 cleavage nuclei first takes place, corresponding to which 

 there is only an indentation of the surface, not a real cleav- 

 age of the egg. This does not take place until there are 

 sixteen cleavage nuclei, when it results in the formation of 

 the same number of separate blastomeres. We see that we 

 here have to do with a variation which forms a transition 



