olio BULLETIN OF THE 



finds reason for the belief that they take their origin from the nuclear 

 structure, — the metamorphosed germinative vesicle and germinative 

 dot. The second polar globule, however, cannot correspond to the whole 

 of the clear space noticed after the formation of the first globule. In 

 this he is unquestionably right, as the clear space, it will not now be 

 doubted, corresponds to that portion of the first archiamphiaster which 

 is not eliminated with the first-formed globule. 



The statement by Dieck ('74, p. 512) that he has recognized the polar 

 globules in the case of decapod crustaceans (Maja and Carcinus) is ren- 

 dered comparatively unimportant by the inaccuracy of his ideas concern- 

 ing the nature of those bodies. According to this author the elimination 

 of polar globules — whose discovery he wrongly ascribes to Johannes 

 Miiller — is to be followed in the nemertean Cephalothrix from the first 

 cleavage onward. They are at first large, but afterwards become smaller, 

 in keeping with the diminution in the size of the segmentation spheres. 

 At length they fill a great part of the space between the embryo and the 

 chorion. This confusion of the production of polar globules with abnor- 

 mal processes has already been pointed out by Biitschli. Dieck, however, 

 observed around the germinative vesicle, and around nuclei generally, a 

 clear zone, and during the reappearance of nuclei after each cell division 

 he mentions that it is in this zone that the new nuclei make their ap- 

 pearance. The " zone " probably corresponds to an aster. 



The unfortunate confusion which Lankester ('74, pp. 375, 376, PI. 

 XYI. Figs. 1-7) experienced regarding the gastrula invagination of 

 Lymnseus was due, in part at least, to not carefully distinguishing be- 

 tween the polar globules and the fluid excretions which are so noticeable 

 a feature of the segmentation stages of pulmonates..* 



In a preliminary note Fol ('74, p. xxxiii.) makes brief mention of the 



* He says: " They [Riehtungsblaschen] may serve a useful purpose for the embry- 

 olo.gist if they enable liim to recognize at any subsequent period when they are pres- 

 ent the original pole at which they made their appearance. But it must be borne in 

 mind that such droplets of albuminous matter are occasionally extruded from eggs of 

 the same character as those of Lymnjeus at other points during later stages in the 

 process of segmentation of the egg sphere." I believe there can be little doubt that 

 Lankester's errors lay in considering the smooth rounded surface shown in his Fig. 4 

 to be the nutritive or less active pole of the egg, and in admitting the possibility of an 

 inconstancy in the relative position of the polar globules in the case of different mol- 

 lusks. Having often seen corresponding stages in Limax, I am convinced that the 

 active pole of the egg is uppermost in his Fig. 4, as well as in Fig. 7, and that his four 

 "large spheres" appear large only because they are very much flattened by the accu- 

 mulation of fluid within. With this explanation there is no serious difficulty in un- 

 derstanding the process of invagination. 



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