514 BULLETIN OF THE 



creased attention it is altogether likely much additional information on 

 this subject may be gained.* 



Polar Phenomena. — Hatschek (77", pp. 524, 525) seems to have 

 been the first to attach a general importance to evidences of polar differ- 

 entiation in early stages of development. It is probable, he says, that a 

 differentiation exists already in the egg cells of all Metazoa. It is most 

 conspicuous in eggs rich in nutritive material, and less evident in small 

 eggs having little nutritive yolk. 



Balfour ('75^ p. 210) had previously called attention to the signifi- 

 cance of a polar concentration of the nutritive portion of the egg in the 

 process of segmentation. " In none of these (vertebrate) yolk-containing 

 ova is the food material distributed uniformly. It is always concen- 

 trated much more at one pole than at the other, and the pole at which 

 it is most concentrated may be conveniently called the lower pole of the 

 egg. In eggs in which the distribution of food material is not uniform, 

 segmentation does not take place with equal rapidity through all parts 

 of the egg, but its rapidity is, roughly speaking, inversely proportional 

 to the quantity of food material." 



It is possible that this accumulation of nutritive substance is not 

 only an expression of the polarity so generally observable, but is also, 

 as it were, a mechanical cause of the condition, since the polarity is de- 

 termined during, and by the method of, the egg's growth. In many 

 cases, at least, — as, for example, in Anodonta, — the acquisition of the 

 food yolk is a one-sided process. It is evident from Flemming's work 

 ('75, pp. 93, 94) that in the case referred to the animal pole is the one 

 w^hich is less charged with nutriment, and perhaps this is because it is 

 more remote from the channel (micropyle) along which commissarial 

 activity is maintained. 



The influence of the nutritive substance upon segmentation and later 

 stages in development was amply comprehended by Haeckel ('74, p. 155, 

 '74", p. 21) at a still earlier period, and has been utilized in his masterly 

 way to explain the superimposed modifications in the process of gastru- 

 lation.f Serviceable as its comprehension has proved in this latter 



* Compare Loven ('48«, PI. X. Fig. 8), Eabl ('76, p. 316, Taf. X. Fig. 4), Hat- 

 schek ('77«, pp. 504, 505, Taf. XXVIII. Fig. 1), and 0. Hertwig {'78°, pp. 202, 

 209, Taf. X. Fig. 5, Taf. XL Fig. 4). 



t "Unter den secunclaren coenogenetischen ErscheinAngen aber, welche den pri- 

 maren paUngenetischen Entwickehmgsgang der Keimformen verdecken und falschen, 

 sind wieder vor Allen wichtig die Einflussreichen Verhaltnisse des Nahrungsdotters 

 im Gegensatz zum Bildungsdotter." — Haeckel '75, p. 404. See also pp. 416-419. 



