No. 3.] MORPHOLOGY OF THE ACTINOZOA. 315 



to enter into a lengthened discussion of the various subordinate 

 and, so far as the essentials of the theory are concerned, unim- 

 portant details, which I have there discussed. 



There is one point, however, which bears directly upon the 

 question before us at present, and that is the origin of delami- 

 nation from immigration. Metschnikoff's explanation of this 

 is not, I think, quite satisfactory. He supposes, in fact, that 

 the two phenomena are contemporaneous, and that both existed 

 in the ancestral Metazoa, the endoderm being formed by "mixed 

 delamination " ; i.e. partly by true delamination and partly by 

 immigration. Where, therefore, in the embryos of higher forms 

 we find immigration alone occurring, there has been a secondary 

 specialization, the delamination having been suppressed ; simi- 

 larly with forms in which delamination alone occurs. It seems 

 to me, however, that, taking into consideration the occurrence 

 of immigration alone in such forms as Volvox and Protospongia, 

 the absence of delamination so far as is known among the 

 Sponges, and its comparatively rare occurrence among the 

 Hydrozoa, since the formation of a solid morula instead of 

 a blastula, such as occurs for instance in Hydractinia, is to 

 be regarded as precocious immigration rather than precocious 

 delamination, — taking these facts into consideration, it would 

 seem that immigration was a more primitive phenomenon than 

 delamination. 



I think a clew to the transformation of the one process into 

 the other is to be found among the Actinozoa, in which, as 

 I have shown, we have variations from such a delamination as 

 occurs in Alcyoniinn, to what we find in Metridiitm. In 

 Reiiilla the protoplasm is so weighted down that the comple- 

 tion of the division plane is difficult. The yolk is segregated 

 towards the central part of the ovum, the periphery being more 

 purely protoplasmic, and in the later stages the nuclei of the 

 cells are in the peripheral portion. By segmentation a Sterro- 

 blastula is formed. There are, consequently, physical difficul- 

 ties in the way of immigration, and, in addition, the food-yolk 

 renders each cell as a whole inert, and deprives it of the energy 

 necessary for immigration. The peripheral protoplasm, how- 

 ever, is unhampered by yolk, and cuts loose from the central 

 yolk-containing portion, a differentiation of the Sterroblastula 

 into a Sterrula thus occurring by delamination. There seems 



