NOTES ON EMBRYOLOGY AND CLASSIFICATION. 411 



hypothesis may be called that of precocious segregation : 

 " precocious/' since it is the acquirement of a condition in the 

 developing organism^ in virtue of heredity, at an earlier 

 period of development than that at which such acquirement 

 was attained by its forefathers through adaptation. The 

 tendency to precocity in this sense, in regard to important 

 structural arrangements, has been insisted on by Haeckel in 

 discussing what he terms " heterochrony in the palingenetic 

 phenomena of ontogeny ;" and the existence of such precocity 

 is as well established as any part of the speculative edifice 

 with which we are dealing, both on a priori and a poste- 

 riori grounds. 



Having, then, arrived at this point, viz. the separation of 

 deric and enteric elements in the first two cells of the de- 

 veloping organism, as a naturally conceivable sequence to 

 the primary process of the separation of these elements by 

 delamination of the walls of a many-celled blastula, let us 

 pursue the case further. 



How, it may be asked, are we to suppose that the enteric 

 and deric cell thus early differentiated should have acquired 

 the faculty of dividing in such a manner that the ofi'spring 

 of the enteric cell form a vesicle which, as it forms, becomes 

 sunk within another vesicle constituted by the deric cells, 

 and that thus the result is a diploblastic Planula or Diblas- 

 tula identical with that formed by Delamination? 



It might be urged that the result of further division on 

 the part of the two primary cells could only be the formation 

 of a vesicular one- cell-layered sac, of the same morphological 

 character as the blastula which precedes the delaminate 

 diblastula, and that we have no suggestion on our present 

 hypothesis of any motive for the invagination of one hemi- 

 sphere of the blastula so developed within the other. We 

 have, however, first of all to note that the blastula (the one- 

 cell-layered sac) belonging to the invaginate series is never 

 precisely the homologue of the blastula belonging to the 

 delaminate series, inasmuch as, according to our hypothesis 

 (and as a matter of actual observation in all invaginate 

 developments), the cells of the blastula belonging to an in- 

 vaginate development are not equivalent one to another, as 

 they are in the blastula of a delaminate development. In 

 an invaginate development from the first the offspring of the 

 primary enteric cell are to be distinguished' from the off- 

 spring of the deric cell, though to the eye there may be no 

 structural distinction (Fig. 8). Accordingly, the blastula of an 

 invaginate development has one hemisphere, or a certain area 

 composed of enteric cells, whilst the rest are deric (Fig, 11). 

 * By means of the ' directive corpuscles,' 



