MUSEUM OF COMPARATIVE ZOOLOGY. 235 



former ; both are structureless. External to the latter is a layer of 

 viscid mucus (Schleim), which fills all the space between the external 

 membrane and the stratified, elastic shell. In approaching the deeper 

 portions of the layer this mucous substance increases in consistency. 



I have been unable to discover the membrana externa, and believe that 

 the structure so named by Warneck may be only a somewhat denser 

 portion of the murcous layer, which does not always differ enough in re- 

 fractive power from the remaining portions of the mucus to make it 

 distinguishable. 



Warneck ('50, p. 108) says that crystals of lime carbonate are found 

 on the outer surface of the outer shell, in some places united into a 

 druse; and subsequently Gegenbaur ('52, p. 372), evidently without 

 knowledge of Warneck's paper, reported that one very frequently sees 

 in the middle strata of the layers of the outer shell deposits of the same 

 substance in the form of dark round concretions, which also often 

 assume a crystalline structure. I have seen nothing of the kind. 



2. The Yolk and its Changes. 



The authors who have studied the early condition and changes of the 

 yolk of Limax are J. L. M. Laurent, Van Beneden and Windischmann, 

 Warneck, and Gegenbaur. 



Very little is to be learned from Laurent ('35^ and '38, p. 136), 

 further than that he believed the vitellus might vary considerably in 

 form, and that it appeared to embrace a variable number (15-20) of 

 large globules containing smaller ones, — views which show clearly that 

 he is more likely to have had under observation segmenting eggs than 

 such as had not reached the cleavage stage.* In view of this fact, not 

 much importance attaches to his statements when he informs us, that a 

 central whitish spot, situated more or less closely to the circumference, 

 is visible by reflected light, and may be due to reflection of the light 

 alone (!) ; that he has never succeeded in recognizing the least indication 

 of a cicatricula produced by the liquid of the germinative vesicle ; or 

 that he thinks there is a vitelline membrane. 



From what has already been said of the view entertained by Van 

 Beneden and Windischmann ('41% pp. 179-181, Taf. 7) as to the 

 nature of the " filament entortille," it will be clear that they inclined 

 to the belief that the freshly deposited egg was without a vitelline mem- 

 brane. All the eggs observed by Van Beneden and Windischmann had 



* The process of segmentation in Mollusks, as is well known, was first announced 

 by Sars ('37, p. 402), in 1837. 



