FORMATION AND FATE OF THE PRIMITIVE STREAK. 83 



At the outset it may, with advantage, be stated that our observations 

 were directed principally to the mode of formation of the archenteron 

 and blastopore, the fate of the blastopore, and the formation and fate 

 of the primitive streak; but incidentally we have dealt with the 

 separation of the germinal layers and the relation of the mesoblast to 

 the chorda and hypoblast. 



We worked independently of each other. One of us was led on to 

 the investigation by the contradictions between the current statements 

 regarding and the obvious facts disclosed by the developing anural 

 ovum ; whilst the other approached the subject with the conviction, 

 based upon theoretical grounds, that either the descriptions of certain 

 phases of Amphibian development were incorrect, or that there were 

 very peculiar and significant differences between the Amphibian and 

 other Vertebrate ova. Until our conclusions had been arrived at 

 neither of us was aware that the other was engaged upon the subject. 

 Consequently our observations were made and our conclusions formed 

 independently of each other. 



Methods. — Living and hardened ova were examined, but we relied 

 chiefly upon sections cut in the three usual planes, horizontally, 

 sagitally, and transversely ; and of these the horizontal have proved 

 in some respects the most useful and instructive, more especially in 

 the observations upon the fate of the primitive streak. 



The ova were hardened either in Perenyi's fluid or in Kleinenberg's 

 picro-sulphuric acid solution. The former fluid was most useful in the 

 younger stages. After the hardening was completed the ova intended 

 for sections were embedded in paraffin in the usual manner. 



None of the ova were stained, for our experience has been that the 

 solutions through which the ova are passed, before the staining is 

 complete, alters the relations of the cells, and that the staining 

 hinders rather than facilitates the microscopical examination of 

 sections of the younger stages. 



We obtained several surface views which were useful for comparison 

 with the results of previous observers ; of these the one represented 

 in fig. 11 was drawn from a living embryo. 



Fig. 23 was drawn partly from a surface view of an ovum hardened 

 in Kleinenberg's fluid, and partly from a model constructed by putting 

 together, in order of sequence, pieces of cardboard cut to correspond 



