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 bands, at any period of their 
history, a lumen, either such as Erlanger has described for Bythinia (92) 
and for Capulus (92%), 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 
multiplieation and accumulation in different regions exercise a profound 
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 eall 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, Blochmann (’81) on Neritina, 
and Rabl (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’2, in the Pteropods (Fol ’75 and Knipowitsch ’91), in Aplysia (Bloch- 
mann 783), and in Fulgur (MeMurrioh *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- 
logieal signifiennee 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 (793), presents the mate- 
