NOTES ON THE DEVELOPMENT OF THE ARA.NEINA. 87 



shows that the whitish hemisphere of the blastoderm is 

 formed of cohimnar cells, for the most part two or so 

 layers deep, but that there is, not very far from the middle 

 line, a wedge-shaped internal thickening of the blastoderm 

 where the cells are several rows deep. A\ith what part 

 visible in surface view this thickened portion corresponds is 

 not clear. To my mind it most probably corresponds to the 

 larger white patch, in which case I have not got a section 

 through the terminal prominence. In the other sections of 

 the same embryo the wedge-shaped thickening was not so 

 marked, but it, nevertheless, extended through all the sections. 

 It appears to me very possible that it constitutes a longitu- 

 dinal thickened ridge of the blastoderm. In any case it is clear 

 that the white hemisphere of the blastoderm is a thickened 

 portion of the blastoderm, and that the thickening is in part 

 due to the cells being more columnar, and, in part, to their 

 being more than one row deep, tliough they have not become 

 divided i?ito two distinct germinal layers. It is further clear 

 that the increase in the number of cells in the thickened 

 part of the blastoderm is, in the maifi, a restdt of the mul- 

 tiplication of the original single row of cells, while a careful 

 examination of my sections proves that it is also partly due 

 to cells, derived from the yolk, having been added to the blas- 

 toderm. 



In the following stage which I have obtained (which 

 cannot be very much older than the previous stage, because 

 my specimens of it come from the same batch of eggs), a 

 distinct and fairly circumscribed thickening forming the 

 ventral surface of the embryo has become established. Though 

 its component parts are somewhat indistinct, it appears to 

 consist of a procephalic lobe, a less prominent caudal lobe, 

 and an intermediate portion divided into about three seg- 

 ments; but its constituents cannot be certainly identified with 

 the structures visible in the previous stage. I am inclined, 

 however, to identify the anterior thickened area of the pre- 

 vious stage with the procephalic lobe, and a slight protu- 

 berance of the caudal portion (visible from the surface) with 

 the primitive cumulus. I have, however, failed to meet 

 with any trace of the cumulus in my sections. 



To this stage, which forms the first of the second period 

 of the larval history, I shall return, but it is necessary 

 now to go back to the observations of Claparede and Bal- 

 biani. 



There can, in the first place, be but little doubt that what 

 I have called the primitive cumulus in my description is the 

 structure so named by Claparede and Balbiani. 



