SUMNER: KUPFFER'S VESICLE 69 



" CONCRESCENCE " as applied to the growth of the embryo, may be taken in two 

 entirely different senses and a failure to recognize this difference may lead to 

 much confusion. As originally conceived by His, 1 concrescence was a process by 

 which the lateral portions of the germ-ring were actually apposed to one other be- 

 hind the embryo, which thus grew backward pari passu with their union. In this 

 case, the embryo was looked upon as formed by the coming together of the two 

 halves of the germ-ring as such, the successive levels in the body of the former 

 being at the outset represented by successively distant portions of the circumference 

 of the latter. The term " concrescence " may be extended in its application, how- 

 ever, to a process quite different from this. The growth of the embryo may still 

 be regarded as taking place at the expense of the germ-ring, but merely in the sense 

 that the latter furnishes undifferentiated building-material, which is first organized 

 after reaching the embryonic region. In this case, the cells composing the opposite 

 halves of the germ-ring are conceived to undergo a gradual concentration toward the 

 axis of the embryo as the latter grows backward. This axial concentration is part 

 of the same general process by which the broad "embryonic shield" gives rise to the 

 narrow, but greatly thickened body of the definitive embryo. It is clear that such a 

 centripetal movement of the germ-ring cells would not necessarily result in a growth 

 of the embryo by accretion at its posterior end. On the contrary, its growth might, 

 in this case, occur by intussusception, the newly added cells reaching the embryo at 

 some point anterior to the caudal end. This second possible mode of concrescence 

 I prefer to call " CONFLUENCE." 



Turning to my own studies, it is evident that I have explained the detachment 

 of the prostoma from the yolk blastopore by a process of concrescence in the former 

 sense (apposition). The duration and extent of this process I have not so far de- 

 termined, but for many reasons I feel convinced that the folding-off of the hinder 

 end of the body is merely the last step in a process of concrescence by which a whole 

 or part of the length of the then-existing embryo has been formed. I am equally 

 convinced that after the detachment of the prostoma and the appearance of caudal 

 knob, concrescence in this sense ceases entirely. For unless we believe this to have 

 come to a close, it would be difficult to account for the continued presence of the 

 projecting caudal knob at the posterior end. Again, if there occurred a union of 

 the halves of the germ-ring behind the embryo, after the formation of Kupffer's 



1 First stated in print in Verh. d. Leipzigcr naturfor.ich. Gen., June 5, 1874. In the same year appeared " Unsere 

 KiJrperform." After this, His reiterated the theo y many times, his last utterance on the subject being in 1891 (see 

 His '91). His was in large measure anticipated by LKUEBOULLET ('63) who distinctly affirmed that the two sides of 

 the embryonic body arose from the two halves of the " bourrelet blaslodermique," or "bourrelet embryngene," as he calls it 

 in view of its supposed fate. Lereboullet did not, however, describe the procesa by which the body arose from the 

 " bourrelet." 



