RAEE FORM OF THE BLASTODERM OP THE OHIOK. 389 



Its diameter gradually diminishes as the Gastrula assumes a 

 more spherical form. Then the double-walled sac elongates in 

 the direction of the axis of the blastopore, and becomes flat- 

 tened on the dorsal side, at the rear end of which is seen the 

 remnant of the blastopore. The lateral edges of the flattened 

 dorsal surface rise up as medullary folds, which enclose at their 

 posterior ends the blastoporic remnant. Considering the matter 

 on the basis that the blastopore has closed up gradually on the 

 dorsal side, it appears not at all improbable that Ave have here a 

 conjunctive form of embryo formation. The investigations of 

 Kowalevsky thus interpreted in harmony with the concrescence 

 theory enabled Rauber to anticipate what has since been veri- 

 fied by observation. Hatschek,^ who has paid special attention 

 to the closure of the blastopore, says, " I came to the conclu- 

 sion that the original broad Gastrula mouth belongs entirely to 

 the later dorsal region, and that one point in its rim marks the 

 hind [end of the body." And again, "The closing of the 

 Gastrula mouth begins at its anterior edge, the 

 hind edge remaining unchanged throughout. The 

 concrescence of the edges takes place in a line that 

 forms the later dorsal line. The hindmost remnant 

 of the Gastrula mouth remains then for some time 

 as a small dorsal opening at the hind end of the 

 dorsal surface." 



The reduction to absurdity claimed in the fourth objection 

 appears to be based on inadmissible premises ; and as soon as 

 these are stripped of exaggeration the absurdity vanishes. 



To say that, according to the view of His and Rauber, the 

 line of concrescence must be coextensive with the whole of 

 the dorsal as well as the whole of the ventral wall of the em- 

 bryo is inaccurate in both particulars. The inaccuracy with 

 respect to the dorsal wall is comparatively unimportant, since 

 only the foremost end of the Elasmobranch embryo undergoes 

 no concrescence, but with respect to the ventral wall tlie 

 statement is ])lainly hyperbolical, and herein lies the whole 



' Hatscbek, " Studicn ii. Eutw. d. Amphioxus," Claus' ' Arbeiten,' vol. iv, 

 part, i, pp. 28, 31, 1881. 



