DEVELOPMENT OF DISTICHOPORA VIOLACEA. 133 



ectoderm has commenced to be differentiated it cannot be 

 observed. In later stages of the development the ectoderm is 

 gradually formed. Nuclei appear in the peripheral sheath of 

 protoplasm, and the protoplasm accumulates in the form of 

 cellular blocks around each nucleus, as in Allopora. 



I have carefully examined the endoderm in these stages in 

 the hope of fiuding out the manner in which the nuclei divide, 

 and although I have found a few dumb-bell-shaped forms, and 

 no satisfactory evidence of karyokinesis, I do not feel justified 

 in asserting that the nuclei always divide amitotically. 



As far as the ectoderm is concerned, I can assert most 

 positively that indirect nuclear division does occur. 



Numerous dumb-bell-shaped nuclei and nuclei connected 

 together in pairs may be seen in the developing ectoderm, and 

 in these faint achromatic lines may be seen connecting the 

 chromatin rodlets. The nuclei are too small to enable me to 

 make out all the details of the process, but there can be no 

 doubt that there is a true process of karyokinesis in the 

 divisions of these nuclei (fig. 18, a, b, c, d, and e). I have not 

 been able to decipher anything like the " spheres of at- 

 traction." 



One very remarkable and important point in the develop- 

 ment of all the Hydrocorallinse, so far as they have at present 

 been investigated, is the fact that there is no segmentation of 

 the ovum, either complete or partial, nor is there any for- 

 mation of cells with a definite outline until a very late stage. 



At the time when (in Allopora and Distichopora) there are 

 ten or fifteen nuclei, the young embryo is a simple multinu- 

 cleated Plasmodium, loaded with yolk. In the later stages the 

 nuclei have increased in numbers, and a certain number of 

 them are arranged in a row at the periphery of the embryo. 



The yolk in the immediate neighbourhood of these peripheral 

 nuclei disappears, probably by absorption, and thus they are 

 situated in a clear peripheral sheath or envelope of protoplasm. 

 In a later stage this peripheral sheath of nuclei breaks up into 

 blocks, each block containing one nucleus, and thus the 

 ectoderm is formed. 



