kofoid: development of limax. 77 



ment of larval organs. In Umbrella this tendency is not so marked, and 

 here definite protoblasts are not distinguishable as early as they are 

 in Nereis. 



The later history of the mesoderm will be discussed in connection 

 with the subject of gastrulation and the fate of the blastopore. There 

 is never developed within these mesoblast bauds, at any period of their 

 history, a lumen, either such as Erlanger has described for Bythinia ('92) 

 and for Capulus ('92 a ), or of any other kind. The bands later lose 

 their distinctness and break up into loose mesenchyma in which it is 

 no longer possible to distinguish pole cells. The mesenchyma cells 

 make their way between the ectoderm and entoderm layers, and by their 

 multiplication and accumulation in different regions exercise a profouud 

 influence upon the form of the embryo. The obliteration of the meso- 

 blast bands by this process renders the determination of the relation of 

 the axes of these bands to the axes of the adult very difficult. 



Inasmuch as both Erlanger ('91) and Heymons ('93) have recently 

 given very full and satisfactory reviews of the conflicting literature on 

 the origin of the middle germ layer in the Mollusca, it hardly seems 

 necessary for me to go over the same ground. It will suffice in passing 

 to call attention to the identity of my results, as to the origin of this 

 layer in Limax, with those of Heymons ('93) on Umbrella, Lillie ('93) 

 on Unio, Conklin ('91, '92) on Crepidula, Blochmanu ('81) on Neritina, 

 and Eabl ('79) on Planorbis, making allowance of course for possible 

 differences in the case of Planorbis due to reversed cleavage. It seems 

 very probable that the mesoderm may have a similar origin, i. e. from 

 d 7 ' 2 , in the Pteropods (Fol '75 and Knipowitsch '91), in Aplysia (Bloch- 

 manu '83), and in Fulgur (McMurrich '86). 



E. Theoretical Considerations. 



The question as to the relation existing between the method of forma- 

 tion of the mesoderm described by Erlanger for Paludina and Bythinia, 

 and the type presented in Umbrella, can find its satisfactory answer only 

 in an examination of these first named forms from the cytogenetic stand- 

 point. As the matter stands now, we are compelled to deny the morpho- 

 logical significance of the precise method of the origin of the middle 

 layer, if we maintain its homology even within the group of the 

 Mollusca. 



The method of origin of the mesoderm in Cyclas, as well as the 

 cleavage according to Stauffacher's description ('93), presents the mate- 



