PRESIDENTIAL ADDRESS. 25 



Iii Renilla, Wilson found an extraordinary range of variation in the 

 segmentation of eggs from which apparently identical embryos were 

 produced. In some cases the egg divided into two in the normal 

 manner, in other cases it divided at once into eight, sixteen, or thirty- 

 two segments, which in different specimens were approximately equal 

 or markedly unequal in size. Sometimes a preliminary change of form 

 occurred without any further result, the egg returning to its spherical 

 shape, and pausing for a time before recommencing the attempt to 

 segment. Segmentation sometimes commenced at one pole, as in 

 teloleeithal eggs, with the formation of four or five small segments, the 

 rest of the egg breaking up later, either simultaneously or pro- 

 gressively, into segments about equal in size to those first formed ; 

 while lastly, in some instances segmentation was very irregular, follow- 

 ing 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 Luinbricus, 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 

 considerable variations in the details of segmentation. 



In the early embryonic stages after the completion of segmentation 

 very considerable variation may occur in allied species or genera. 

 Among Ccelenterates, for instance, the mode of formation of the 

 hypoblast presents most perplexing modifications : 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 start as a solid mass 

 of cells enclosed by the epiblast. It is in connection with these 

 variations that controversy has arisen as to the primitive mode of 

 development of the gastrula, a point to which I shall return later on. 



Among the higher Metazoa or Ccelomata the extraordinary modifica- 

 tions in the position and every conceivable detail of formation of the 



