167 



and Sedgwick, regarding the importance of the relation of the blastopore 

 to other parts are as forcible as ever. 



Among the Annelids and those groups (Arthropods, Molluscs, etc.) 

 which in various adult anatomical features shew indications of genetic 

 relationship there is very distinct embryological evidence of an elong- 

 ation of the original radial organism in such a way as to draw out 

 the original blastopore into a slit and perhaps giving rise to both 

 mouth and anus in the ancestors of all these forms. It is not neces- 

 sary to do more than to refer the reader to Sedgwick's paper on 

 this point. 



GoETTE, in his recent text-book of Zoology authoritatively em- 

 phasises the fact that in no vertebrate is there any real trace of the 

 blastopore becoming mouth, and has grouped the Chordates with the 

 other great phylum in which the same fate of the blastopore is estab- 

 lished — the Echinodermata, an association which is supported by 

 other important features of the very early developmental history known 

 to all Zoologists (cf. Morgan, Journ. Morph., Vol. 5, Mac Bride, 

 Quart. Journ. Microsc. Sc, Vol. 38). 



In the Echinoderms, however, there is no evidence that the deutero- 

 genetic centre ever arose — or at least if it arose it never attained 

 to any importance, with the result that in them there is retention, or 

 if it departed from it for a short time a return to, a radial symmetry 

 and absence of what we call metameric segmentation. 



Lastly, the direct support from embryology which Sedgwick's 

 theory of an elongated blastopore for Vetebrates seemed to receive 

 was the supposed formation of the embryo or of the primitive streak 

 by an actual concrescence of the germ ring as propounded by His. 



Any unbiassed student of the experimental work performed by 

 Kastschenko, Morgan, Kopsch, Sumner, Peebles and others, in- 

 cluding myself, must come to the conclusion that the concrescence 

 theory as propounded by His rests upon the slenderest evidence; it 

 is in my opinion quite untenable. 



To return again to the question of the meaning of the term 

 "gastrulation". 



The essence of Keibel's definition (quoted by Hubrecht) appears 

 to be the attainment of the centre of the germ by certain cells, im- 

 plying a motion on the part of these cells: "Die Gastrulation ist der 

 Vorgang, bei dem das Material in das Innere des Keimes gelangt." 



It is quite true that actual motion does in some cases occur — 

 e. g., during invagination of the Amphioxus blastula, but this is of 

 quite secondary importance. To me it seems that the essence of 



