236 BULLETIN OF THE 



been deposited for a longer or shorter time. In keeping with the then 

 prevailing opinion of the total disappearance of the germinative vesicle, 

 they report not being able to find a trace of it at tlie centre of the 

 yolk. The first change observed was the appearance of a transparent 

 vesicle, which seemed to escape from the midst of the yolk, and was 

 soon followed by a second. These vesicles (polar globules) are never 

 wanting, and always escape from the same side of the yolk. After their 

 escape, the space which they have traversed appears clearer (deep aster 1) 

 than the rest of the vitellus (sufficient cause for them to institute 

 a comparison with the vase-shaped portion of the white yolk in the 

 fowl's egg). The vesicles contain granules some moments after their 

 escape ; they take no part in the formation of the embryo, but are sub- 

 sequently absorbed in the albumen. The observers are unable to say 

 whether these vesicles have any analogy with the germinative vesi- 

 cle, but think it in no way astonishing that the latter should escape 

 from the yolk, if the vitelline membrane has suff'ered the change above 

 alluded to, although the vesicle would in that event cease to have the 

 important (germinative) role attributed to it in higher animals. 



What is said concerning the first segmentation becomes intelligible 

 only by comparing with the figures, and apparently rests on a miscon- 

 ception of the order of events. It is stated (p. 181) that, after the escape 

 of the two vesicles, the middle of the vitellus becomes clearer, and is 

 divided into two equal portions, and that subsequently a furrow makes 

 its appearance at the side opposite that where the (polar) vesicles es- 

 caped. An examination of Fig. 5 (Taf. 7), which is described (p. 194) 

 as representing the stage in which the yolk has become clearer in 

 the centre, shows conclusively, to one who has seen the object itself, 

 that the clearness in question is due to a lense-shaped accumulation 

 of a transparent fluid between the cleavage spheres,* and that con- 

 sequently the figure represents a stage after the first cleavage. I am 

 unable to explain how it happens that the cleavage furrow should be 

 considered as first appearing at the side opposite that where the polar 

 vesicles arise (see Fig. 7). That the process was not very carefully ob- 

 served is moreover sufficiently demonstrated by this Figure 7, for it un- 

 questionably represents an egg after the first segmentation, and after 

 the accumulation of the lenticular mass of fluid above alluded to. 



Warneck ('50, pp. 102, 103, 105, 114) thinks that the yolk in the 

 eggs of Lymnseus and Limax has no special envelope, but is simply 



* This interesting phenomenon has heen in most points already well described by 

 Warneck, and has been seen in other than moUuscan eggs. 



