8S MCMURRICH. [Vol. XI. 



it is certain, I think, from the sections I possess that they 

 exist, yet I have not been able to get a clear idea from Jacra 

 either as to their arrangement or as to their origin. 



The development of Jacra has now been traced up to a 

 period in which the germ layers have assumed their general 

 distribution. Further changes are mainly of a histogenetic 

 character and may be postponed for the present until an 

 account has been given of the development of Aselhis, Porcellio, 

 and Armadillidium. 



2. Tlie Segmentation and Formatiott of the Germ-layers of 



Aselliis Commtmis. 



So far as I am aware, there are no published descriptions of 

 the early development of the common Aselhis of this country, 

 first described by Say as Aselhis communis, though several 

 more or less complete accounts have been given of the Euro- 

 pean A. aquaticus. Rathke ('34), Dohrn ('67), and Sars ('68) 

 did not make any definite observations upon the segmentation, 

 but Van Beneden ('69) describes accurately its general features, 

 showing that it is of the typical centrolecithal character and 

 regular, though he did not attempt to follow it out in detail. 

 The latest author who has written on the subject, Roule ('89), 

 has certainly not advanced our knowledge of it. His state- 

 ments in the brief contribution he has published are so remark- 

 able and so little in harmony with those of Van Beneden and 

 with what might be expected from analogy with other Crus- 

 tacea, that one involuntarily distrusts them. They are to be 

 explained probably as due to failure to study in surface views 

 stained ova, the only method by which the phenomena of the 

 early stages can satisfactorily be made out. 



My observations on Aselhis commimis are, I regret, not quite 

 so complete as these on Jaera, inasmuch as I failed to obtain 

 a few stages; they are, however, sufificiently connected to allow 

 of a close comparison with Jacra, which was the object I had in 

 view in undertaking them. The early segmentation is step for 

 step identical with that occurring in Jaera, so far as that form of 

 segmentation followed is concerned. I have reason to believe, 



