EMBRYOLOGY OF CLEPSINE. 307 



(sVtt) ^'^^^ by His (tMj)' Kowalevsky (-pVV) 'i^s found the 

 same conuection in Accipenser and the frog, Bobretzky (t-t^,-) the 

 same in Axolotl, Max Schultze ( '-fV) ^^^ Owsjannikow the same 

 in Petromyzon, Kupffer and Kowalevsky the same in Ascidia and 

 Amphioxus (^Vo)- Recently (Oct. 26^ '78) the discovery of the 

 same connection in the bird has been reported by Gasser (51). He 

 says, " Bei Giinse-Embroyonen von ungefahr 17-~0 Urwirbeln 

 besteht an einer bestimmten Stelie der Schwanzansschwellung 

 ehie offene Communkaiioii des Centrabiervenrohrcs mit dem 

 Lumen der Chorda und dem Entoderm, also das Entoderm seizt 

 sich direct in das Ectoderm, Centrulnervenrohres fort." Should 

 this be confirmed it will be ouly a new and convincing 

 evidence of the relationship between the neurula of the chick 

 and that of the fish, and at the same time of the correct- 

 ness of Eauber's interpretation of the marginal notch {m.n.) 



In comparing the neurula of Clepsine with that of Vertebrates, 

 an interesting question arises in regard to the cause of Ihe 

 central thinning of the blastodisc and the concomitant foruiatiou 

 of a marginal rim or band. Docs the embryonic rim thicken 

 only relatively ? or does it thicken absolutely ? Intimately con- 

 nected with this question is that in regard to the origin of the 

 primitive streak. These questions have long engaged the atten- 

 tion of embryologists, but have not yet been fully answered, 

 especially in the case of discoidal develo])ment.^ As a contribu- 

 tion to the solution of these problems, Professor His (76) has 

 given a large number of embryometrical tables from which it is 

 plain that the embryonic rim (cliick) not only increases in sur- 

 face but in depth, and that it is thicker in the posterior (" retro- 

 central '') than in the anterior (" prre-embryonal '') region. 

 The latter fact indicates that the posterior region is the place of 

 most energetic growth or concentration ; and this is what we 

 should expect if the development here is comparable with tlie 

 concrescence of germ-bands. 



According to Bauber ( ^ .]: ^ 3-) the primitive streak results from 

 a concentration of the " entodermic lunula ""^ to both sides of the 

 longitudinal axis of the future embroyo — in other words — it is 

 a " phenomenon of conjunction." The forward growth of the 

 streak, both in duration and extent, is unimportant as compared 

 with the growth in the opposite direction, which takes place, in 

 the mam, by a conjunction (•'•' association") of the two lateral 

 halves of the embryonic rim. Disse (yi/') puts the matter in 

 the same light when he says. " diese Verdickung (primitive 

 streak) entsteht durch centripetale Zcllverschiebungin der unteren 

 Keimschicht aus dem Ptandwulster." Thus the embryo chick 



1 Balfour (12) was the first to suggest the ideiitiiy of the primitive streak 

 wilb a part of the blastopore. 



