452 ARTHUR ROBINSON AND RICHARD ASSHETON. 



development, were it not that we think the results of our recent 

 observations on the development of the Anura tend to simplify 

 the problem we have been studying by throwing light upon 

 several interesting developmental phenomena. For these 

 reasons alone are we induced to publish the results of a research 

 which tend to conclusions difiFerent from those of our prede- 

 cessors, whose experience is, in most cases, much more exten- 

 sive than our own. 



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

 servations 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 de- 

 velopment were incorrect, or that there were very peculiar and 

 significant differences between the Amphibian and other Verte- 

 brate 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 conclu- 

 sions 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 hori- 

 zontal have proved in some respects the most useful and in- 

 structive, 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. 



