232 BULLETIN OF THE 



that it is only incompletely differentiated from the underlying granular 

 protoplasm it is found in places to have portions of the latter clinging 

 to its inner face. From this I can hardly conclude otherwise than that 

 a differentiation has already begun in the superficial portion of the yolk, 

 which is the first step toward the formation of a cell membrane, and that 

 this differentiation is proportional to the advance of the cleavage. It 

 is unquestionably owing to the action of the acid that this layer be- 

 comes detached from the vitellus ; but it seems to me unnatural to con- 

 clude that the homogeneous layer has itself been produced by the action 

 of the reagent. 



B. BIBLIOGRAPHY. 



I. LiMAX. 



The embryology of Limax has been studied by a number of natu- 

 ralists. The most of the papers on this subject, however, were written 

 many years ago ; indeed, no extensive contribution to the embryology 

 of Limax has appeared for the last quarter of a century. The phe- 

 nomena which are considered in the present paper were in many cases 

 either briefly touched upon, or fell altogether outside the province of the 

 authors' investigations. Of the latter class, and consequently of less 

 immediate concern at present, are the papers of J. L. M. Laurent ('37, 

 '37^ 37^ and 38'*) ; P. Laurent ('42) ; Dujardin ('37) ; Schmidt ('51) ; 

 Gegenbaur ('52) ; and Lankester ('74 and '75^).* 



1. Egg Envelopes, etc. 



The composition of the egg at the time of extrusion has been studied 

 by several writers, whose conclusions, though not difficult to interpret, 

 are somewhat at variance. 



The earlier descriptions date from a time when the process of segmen- 

 tation was as yet unknowm in Mollusks. It would be unreasonable to 

 expect from them valuable observations on many of its structural 

 features. 



Turpin's ('32, p. 435) description is substantially as follows. The 

 eggs of Limax flavus (Fig. 9) are oblong, terminated at each end by a sort 

 of umbilical chord ; they are transparent, bluish or grayish, soft and gelati- 

 nous. The eggs of the Limaces are composed, like those of Helices, of 



* The numbers in Egyptian type following an author's name are abbreviations 

 for the year of publication, — e. g. '37, '37^ '37% all appeared in 1837, — and sei-ve 

 at the same time to refer the reader to the alphabetical list of authors quoted. 



