458 MORGAN. [Vol. X. 



decreases from 1.2 mm. to 0.55 mm., i.e., as 100 to 40. His 

 does not believe, apparently, that this has anything to do with 

 the elongation of the embryo posteriorly. He extends his 

 theory of concrescence to higher groups, and rejects the con- 

 ception of a coincidence of blastopore and germ-ring in Saurop- 

 sida and mammals (see Hertwig (3)). 



In the discussion that followed His' paper (6), in 1891, 

 Riickert (15) states that he believes that material from the 

 germ-ring passes into the embryo (" in die axiale Anlage auf- 

 genommen werde "). He rejects the results of Kastschenko's 

 experiment (7), because he has himself also cut the germ-ring 

 of Pristiurus at one side of the embryo, and has carried such 

 embryos to a later stage then did Kastschenko. He finds 

 " eine geringere Ausbildung resp. einen Defect auf der ope- 

 rierten Seite (bei oberflachen Betrachtung gesehen)." 



Sedgwick, in 1892 (16), has described the Elasmobranch 

 development and given his idea of the meaning of concres- 

 cence. He says : " It must be clearly understood that the 

 growth of the whole edge of the blastoderm has so far been a 

 uniform one. The indentation in the embryonic rim advances 

 equally with the more prominent parts of the embryonic rim 

 called the caudal swellings. There is no reason to suppose 

 that this advance of the indented part of the embryonic rim is 

 due to the fusion of the divergent caudal swellings. On the 

 contrar}^, there is every reason to suppose that the indented 

 part of the embryonic rim advances by growth of its own sub- 

 stance, just as do the other parts of the edge of the blasto- 

 derm." "Further, it is clear from what I have said above that 

 the notch of the embryonic rim represents the anterior end of 

 the blastopore, and that, on the view of embryonic growth 

 above stated, the blastopore does at one time or another per- 

 forate the whole length of the medullary plate. . . . Anteriorly 

 it keeps closing up as the embryonic rim grows backwards, so 

 that it is never present in this region as more than a notch." 

 " It will be maintained by some that this view of the growth 

 of the embryo aj;id of the relation of the blastopore to the 

 medullary plate is incompatible with the objection to the con- 

 crescence theory above formulated. To this the reply would 



