782 THE AMERICAX XAri RAUST. [Vol. XXXVIIl. 



The General Featl rks of thk Fckmation of the Head 

 OF Blatta. 



The changes during the first six days i of the embryonic hfe 

 of Blatta lead to the formation of the ventral plate, the blasto- 

 dermic thickening which represents the first rudiment of the 

 germ band. This extends along about two-thirds of the ventral 

 surface of the egg, beginning at about one-fourth of the length 

 of the egg from its cephalic end. 



The newly formed germ band is not an evenly developed struc- 

 ture, but there are to be seen, on each side, groups of rapidly 

 proliferating cells. These active cells give rise to an indistinct 

 segmentation in the earlier stages of the germ band. This was 

 noted by Cholodkowsky ('89), who says, (p. 91): "However, 

 when I say that the young germ band of Blatta gives no indica- 

 tion of cross divisions I do not mean by that that there are no 

 traces of metamerism. Very early when the germ band is still 

 very imperfectly marked off from the surrounding undifferen- 

 tiated blastoderm, one may observe a noteworthy grouping of its 

 cells around certain points w^hich are none other than the centers 

 tor the formation of the future appendages." 



Heymons ("95), has correctly interpreted this description of 

 ("liolodkowsky's as referring to-certain formative centers in the 

 unditfoixMUiatcd blastodern-i, which by their extension and fusion 

 ,^ivc lisc to the germ band. A similar method of formation of 

 llH' ,L;orm hand of Astacus had already been i)ointed out by 



