300 C. O. WHITMAN'. 



sometimes arise with the ectoderm, and so give the appearance of 



arising />f/M the ectoderm fBobretzky, -ttV)- 



{cl) The Gastrula. 



Supported by the investigations of the celebrated Russian 

 embryologist, Kowalevsky, as well as by their own observations, 

 Hackel and Lankester arrived, independently of each other, at 

 fundimentally similar views in regard to the importance and 

 universality among Metazoa of the Gastrula or Planula-phase of 

 development. Both these investigators published sketches of 

 their views at about the same time (61, 96), Hiickel's appearing 

 but a little earlier than that of Lankester.^ The latest form of 

 Hackel's theory appeared in the ' Jenaische Zeitschrift/ vols, 

 viii, and ix. (64, 65). A complete view of Lankester's Planular 

 theory, and its points of divergence from the Gastrula theory 

 was published in this journal, Oct. 1877. 



The chief points of difference between the two theories in their 

 latest forms concern the interpretation of the Gastrula orifice 

 (blastopore, Lankester), and the genealogical relationship between 

 the delaminate and invaginate forms. Hackel has from the 

 outset adhered to the opinion that the " gastrula iuvaginata^' 

 (Lankester) is the primitive form, and the " gastrula delaminata,'' 

 if such exist, a secondary form which has arisen by cenogenetic 

 changes from the former ( -yVV-). What these changes are, or 

 in what conceivable way the one form could pass into the other, 

 Hackel does not attempt to say. Manifestly there is some diffi- 

 culty here. 



Lankester, who at first entertained the same opinion, has in 

 his last ])aper (101) strongly advocated another view, viz., that 

 the delaminate Planula is tlie primordial form. Lankester has 

 undertaken to account for the substitution of invagination for 

 delamination on the hypothesis of "precocious segregation.^'' 

 Ihis principle, which he has recognised in former papers, but 

 which is here for the first time clearly formulated, and its appli- 

 cation to the question under consideration, will be best under- 

 stood if stated in the author^s own lucid words. '' Though the 

 substance of a cell may appear homogeneous under the most 

 powerful microscope, excepting for the fine granular matter sus- 

 pended in it, it is quite possible, indeed certain, that it may 

 contain, alreadij formed and individualised, various kinds of 

 physiological molecules. The visible process of segregation is 

 only the sequel of a difierontiation already established, and not 

 visible. Tiie descendants of the Diblastula (diploblastic Planula), 



' See also " Development of llio Poiul-Siiail," this Journal, vol. xxii 

 (n.s. 14), pp. 365—307 



