S02 C, O. \VH1TMAX. 



factors is only completed after the so-called morula stage is 

 reached. 



In Clepsine ectoplasm and endo])lasm are iiot fully segregated 

 till about the time the germ-bands begin to form. 



Without citing further examples it may be stated, as a general 

 fact, that the segregation of ectoplasm and endoplasm goes on 

 duriticj cleavage as well as previous to cleavage. In the more 

 typical forms" of invagination this differentiation manifests itself 

 later than in the modiUed forms of epibolic invagination. Indeed, 

 in some cases there is no appreciable differentiation till after the 

 invagination begins. 



But how is it in the case of delamination ? According to 

 Fol (40, Geryonia) the ectoplasm is distinct from the endoplasm 

 before the cleavage begins, and remains distinct during the whole 

 ])criod of cleavage. To be sure, the dejiniiive separation of one 

 from the other is accomplished with the formation of the Planula. 

 In what case of typical invagination do we find so complete pre- 

 cocious segregation ? A similar precocious differentiation is 

 manifested in some other Ccelenterata {e.g. EschoUzia cordata, 

 Kowalevsky, 84), where a more or less modified form of invagina- 

 tion takes place. Such cases do not, however, lessen the force 

 of the above objection. The (ivaUiaUve and topical differentiation 

 of elements which characterises the egg of Geryonia appi'ars to mc 

 irreconcilably opposed to the hypothesis of primary delamination. 



Another objection is the fact that the delaminate Planula is 

 reached by a more direct route than the invaginate planula, and 

 hence bears the mark of ontogenetic abridgement. 



Still another objection is the occurrence of the invaginate 

 development in the very lowest Metazoa (Dicyemida, , .] ^ , ; 

 Gastrophysema, -f4{;-; Sycandra raphanus, -.Vfi )• 



A fourth objection is found in the progressive differentiation 

 of the primary lamellae during cleavage, and especially during 

 the invagination. The theory of priuiary invagination disjioscs 

 of these objections and furnishes an easy explanation of the 

 primary differentiation into ectoderm and entoderm. 



Theprimarycauseof invagination is undoubtedly the cause which 

 operates to-day. This cause, so plainly seen in epibolic invagina- 

 tion, is t/ie unequal grotvth of the two poles (hemispheres) of the 

 Blaslula. This is well illustrated in that rare but instructive form 

 of the IJlastuia described by Biitschli (-j-— ^.", ^ - „ -, C'ueuUanus). 



15efore the invagination as Putschli remarks (j). 10(5), the 

 character of the two cell-layers is the same. The process of 

 invagination is thus described : — "As the outer eel. -layer (ecto- 

 derm) enters upon a rapid growth in which the future inner 

 layer (entoderm) does not ])artiei])atr, the cell-plate begins to 

 bend, becomes liollow, and liiially the edges close over the cavity," 



