296 GEORGE W. BARTELMEZ 



torial plate stage of the second maturation spindle is found. It 

 may be said of the pigeon's egg, then, as of every other well 

 established case in the vertebrates, that the egg does not proceed 

 beyond the metaphase of the second maturation unless it be 

 fertilized. It agrees also with most (perhaps all?) vertebrate 

 eggs in that normally the first polar body is given off in the ovary, 

 and ovulation takes place while the second spindle is in metaphase. 

 I have no explanation to offer for the definite relation between 

 the breaking down of the germinal vesicle and ovulation. Taking 

 all the data into consideration, however, it would seem that the 

 maturation processes begin soon enough so that they may progress 

 as far as the middle of the second maturation and rest there until 

 the yolk secretion and the other factors have brought about 

 the rupture of the follicle. The latter may take place during the 

 first maturation, judging from the fertilized egg figured and 

 described by Harper, '04, figures 6 and 6a, still I am convinced 

 that the account given above describes the usual course of events, 

 since Harper's is the only similar case that has been found. 



V. THE BILATERALITY OF THE BLASTODISC 

 (Summary of Part II) 



A. Origin 



The only reference in the literature to the origin of the blasto- 

 disc in the bird's egg is, so far as I know, Coste's ('47) surmise 

 that it arises from 'le contenu granuleux' (the spherule cap, p. 

 277, ss.), but the observations of Agassiz and Clark ('57) on the 

 turtle egg are of value in connection with the conditions described 

 here. It has been shown that the spherule cap of deutoplasmic 

 granules is used up during the period of differentiation of the 

 oocyte (p. 286). The first traces of a blastodisc appear toward 

 the end of this period of development, when the oocyte is about 

 2 mm. long axis (in life), and when the germinal vesicle has begun 

 to flatten out against the follicular epithelium. Figure 26 shows 

 a longitudinal median section of an oocyte in this stage and figure 

 28 is from the animal pole of the same oocyte more highly magni- 

 fied. It will be noted that the reticulum, the spaces of which 



