1884.] 125 [Hyatt. 



defect in observation), the true hydroplanula is the apparently 

 two-layered form immediately succeeding the gastrula (f. 3) ; but 

 both parenchymula and hydroplanula are probably mucli abbrevi- 

 ated. 



The difficulties presented by the change from invagination 

 to delamination have been met by Lankester with the aid of 

 hypothetical molecules of protoplasm respectively called deric 

 and enteric, which were supposed to have acquired the develop- 

 mental tendencies of the primitive membranes in which they 

 originated. Thus the gastrula was imagined to have become 

 changed by abbreviation of development into the planula, and 

 this process was supposed to have been rendered possible by the 

 existence of peculiar endoblastic, enteric molecules, which were 

 finally transferred by these changes from the separate cells of the 

 endoblast in which they arose to the inner poles of the primitive 

 ectoblastic cells of the planula. 



We should prefer to seek for the solution of this problem in the 

 relations of the planes of fission to the axes of the body, or of the 

 cells themselves. The planes of fission by which the cells of the 

 endoblast in the amphimorulae of Porifera are differentiated may 

 have been parallel with their longest axes, or more or less 

 inclined to them, but were probably not transverse. On the 

 other hand the planes of fission in the delamination of the 

 mesenchyme cells habitually appear to be transverse, and in Gery- 

 onea (Fol) and in an unpublished plate of the embryo of Eutima 

 by Prof. W. K. Brooks equally decisive cases of delamination 

 of the ectoblast by transverse fission occur, 1 In all instances the 

 first stage of differentiation by fission would necessarily result in 

 the formation of an endoblast, and it is quite within the limits of 

 possibility that extra growth or quicker and earlier growth of 

 the ectoblast may have been the immediate cause of the origin of 

 the endoblast from the inner poles of the ectoblastic cells by 

 transverse instead of by radial fission. 



This explanation we rejoice to notice has been anticipated by Dr. 

 C. O. Whitman (op. cit., p. 303), so far as relates to the formation 

 of the gastrula, and this gives us necessarily greater confidence in 

 its general application to the origin of invaginations. Whitman's 

 remarks are as follows : " Starting with a typical blastula it is not 

 i See also Zool. Anzeig., 1884, vol. vn, p. 709. 



