THE RECAPITULATION THEORY 327 



returning to its spherical shape, and pausing for a 

 time before recommencing the attempt to segment. 

 Segmentation sometimes commenced at one pole, as 

 in telolecithal eggs, with the formation of four or 

 five small segments, the rest of the egg breaking 

 up later, either simultaneously or progressively, 

 into segments about equal in size to those first 

 formed ; while lastly, in some instances segmenta- 

 tion was very irregular, following no apparent law. 

 It is noteworthy that the variability in the case of 

 Renilla is apparently confined to the earliest stages, 

 for whatever the mode of segmentation, the embryos 

 in their later stages were indistinguishable from one 

 another. Similar modifications in the segmentation 

 of the egg have been described in the oyster by 

 Brooks, in Anodon and other Mollusca, in Hydra, 

 and in Lumbricus, in which last Wilson has recently 

 shown that marked differences occur in the eggs 

 even of the same individual animal. In the different 

 species of Peripatus there appear also to be con- 

 siderable variations in the details of segmentation. 



In the early embryonic stages after the comple- 

 tion of segmentation very considerable variation 

 may occur in allied species or genera. Among 

 Coelenterates for instance the mode of formation of 

 the hypoblast presents most perplexing modifica- 

 tions : it may arise as a true gastrula invagination ; 

 as cells budded off from one pole of the blastula 

 into its cavity ; as cells budded off from various 

 parts of the wall of the blastula ; by delamination 

 or actual division of each cell of the blastula wall ; 

 or it may be present from the first as a solid mass 



