The Morphology of C'osinobia 433 



At the end of the paper Kaestner discusses numerous cases of nuiltiitle 

 embryonal areas upon a single 3'Olk, a field which is of the utmost 

 importance, especially with reference to the formation of unecpial dip- 

 lopagi (autosite and parasite). He accords with severa] other writers 

 in emphasizing the presence of multiple cleavage centers, "mehrfache 

 Furchungszentren," as the earliest stages of multiple formations. These 

 may differ widely in their distance from and tlieir geometrical relation 

 to one another, ranging from those placed at diametrically opposite 

 poles to those that are so near that the two cleavage systems interlock, 

 "dass die beiden Systeme von Furchen jetzt schon ineinander greifen." 

 Wlien these two cleavage centers are far apart they lead to the develop- 

 ment of two separate germinal discs, but whether in later development 

 their edges remain separate or become united we do not know. From 

 a preparation of Miteaphakow, in which, in a six-hour chick, there 

 are two are* pellucida? upon a single "biskuitformige" germinal disc, 

 the impression is given that here two originally separate germinal discs 

 have fused, although the sections show that ectoderm and endoderm 

 pass in direct continuation across the dividing line from the one to the 

 other. This latter observation strongly suggests that mo secondary 

 fusion has taken place and that the material of the two has been con- 

 tinuous from the first, "also scheint das Material von Anfang an 

 zusammengehangen zu haben." In either case, whether there was a 

 secondary fusion or not, Kaestner concludes that there must have been 

 originally two cleavage centers, although rather nearer together than in 

 case? in which there are two separate germinal discs. From this, cases 

 like his Fig. 15, with two closely adjacent areas pellucidse, can be readily 

 explained, and a still closer approximation of the cleavage centers would 

 result in a form in which the arege pellucidge would be themselves con- 

 fluent. From this Kaestner draws the conclusion that cleavage centers 

 placed far apart lead to two independent embryonal anlagen, while, if 

 we imagine them as gradually approximating each other, there comes a 

 point beyond which they are so influenced by each other that they form 

 a symmetrical double formation. 



Finally he cites several well-known multiple embryos and suggests 

 what they might have become, judging from the relations of the separate 

 anlagen. Wetzel's case, with four centers, is of special interest. The 

 centers are associated in pairs, A+B and C-f D, of which A and B are 

 nearer together than are C and D. Kaestner thinks that the former 

 have already come into collision, and that they would lead to two 



