NOTES ON THE DEVELOPMENT OF THE ARANEINA. 673 



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, though they have not become divided into 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 

 main, a result of the multiplication 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 blastoderm. 



In the following stage which I have obtained (which cannot 

 be very much older than the previous stage, because my speci- 

 mens 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 proce- 

 phalic lobe, a less prominent caudal lobe, and an intermediate 

 portion divided into about three segments ; but its constituents 

 cannot be clearly identified with the structures visible in the 

 previous stage. I am inclined, however, to identify the anterior 

 thickened area of the previous stage with the procephalic lobe, 

 and a slight protuberance 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 Balbiani. 



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

 have called the primitive cumulus in my description is the struc- 

 ture so named by Claparede and Balbiani. 



It is clear that Balbiani and Claparede have both failed to 

 appreciate the importance of the organ, which my observations 

 shew to be the part of the ventral thickening of the blastoderm 

 where two rows of cells are first established, and therefore the 

 point where the first traces of the future mesoblast becomes 

 visible. 



Though Claparede and Balbiani differ somewhat as to the 

 position of the organ, they both make it last longer than I do : 

 I feel certainly inclined to doubt whether Claparede is right in 

 considering a body he figures after six segments are present, to 



