412 PROFESSOR LANKESTER. 



Now, we must remember that, however difficult it may be to 

 form a mechanical conception of the processes by which the 

 cells derived by division from an enbryonic cell take up certain 

 definite positions, so as to form definite organs comparable to 

 those of the parent organisms (thus exhibiting what we call 

 heredity), it is no more difficult to form a mechanical con, 

 ception of this power of self-arrangement and co-ordination- 

 as it exhibits itself after a certain amount of interference 

 with the routine of recapitulative heredity, than as it exhibits 

 itself when that routine is uninterruptedly pursued. In 

 virtue of an hereditarily transmitted molecular structure 

 the cells formed by division of the egg-cell in a delaminate 

 development arrange themselves as a sac, the blastula. In 

 virtue of hereditarily transmitted molecular structure the 

 offspring of the enteric and the offspring of the deric cells, 

 which are diflPerentiated in the first cleavage of the egg of 

 an invaginate development, arrange themselves as two vesi- 

 cles, the latter within the former, the two groups of cells, 

 each reproducing from the first, the characters of the endo- 

 derm and ectoderm of the parental Diblastula, in regard to 

 such points 3S plane of cleavage, contact of the cells of one 

 layer with one another, and contact with those of the other 

 layer. Accordingly, the immediate apposition of the endo- 

 dermal to the ectodermal cells (such as often occurs, e.g. in 

 mammals, in nematods, and in the earthworm), without the 

 formation of a vesicular blastula, is what we should look for 

 (Figs. 9, 14). 



The formation of a vesicular blastula in the course of an 

 invagi7iate development is a secondary j)yocess : such a blas- 

 tula (Fig. 11) is not the representative of the ancestral 

 blastula (Fig. 3) which appears in the course of the delami- 

 nate development; it is due to mechanical non-hereditary 

 accumulation of liquid among the primary cells of the embryo, 

 and distorts the recapitulative development. The blastula of 

 the invaginate development may be called a ' pseudoblastula' 

 to distinguish it from the ancestral blastula. Haeckel's archi- 

 blastula is a pseudoblastula, and its cavity does not correspond 

 with the cavity of the delaminate blastula, which immediately 

 becomes the archenteron. We distinguish the archenteric 

 blastocoel from the pseudoblastocoel. 



The space which is formed within or is enclosed by the 

 products of the enteric primary cell is, of course, the homo- 

 logue of the blastocoel of delaminate development. It is this 

 which becomes the archenteron. As the multiplication of 

 the deric and enteric cells goes on, the cavity enclosed by 

 the enteric cells becomes more distinct. The margin of the 



