^8 f. M. BALFOUft. 



It is clear that Balbiani and Claparede have both failed to 

 appreciate the importance of this organ^ which my observa- 

 tions show 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 meso- 

 blast become 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 be the same as the dorsal organ of the embryo 

 before the formation of any segments, especially as all the 

 stages between the two appear to have escaped him. In 

 Agelena there is undoubtedly no organ in the position he 

 gives when six segments are formed. 



Balbiani's observations accord fairly with my own up to 

 the stage represented in fig. 2. Beyond this stage my own 

 observations are not satisfactory, but I must state that I feel 

 doubtful whether Balbiani is correct in his description of the 

 gradual separation of the procephalic lobe and the cumulus, 

 and the passage of the latter to the dorsal surface, and 

 think it possible that he may have made a mistake as to 

 which side of the procephalic lobe, in relation to the parts 

 of the embryo, the cumulus is placed. 



Although there appear to be grounds for doubting whether 

 either Balbiani and Claparede are correct in the position 

 ihey assign to the cumulus, my observations scarcely warrant 

 ir.e in being very definite in my statements on this head, but, 

 as already mentioned, I am inclined to place the organ near 

 the posterior end (and therefore^ as will be afterwards shown, 

 in a somewhat dorsal situation) of the ventral embryonic 

 thickeniiig. 



In my earliest stage of the third period there is present, 

 as has already been stated, a procephalic lobe, and an indis- 

 tinct and not very prominent caudal portion, and about 

 three segments between the two. The definition of the 

 parts of the blastoderm at this stage is still very imperfect, 

 but from subsequent stages it appears to me probable that the 

 first of the three segments is that of the first pair of ambu- 

 latory limbs, and that the segments of the chelicerae and pedi- 

 palpi are formed later than those of the first three ambula- 

 tory appendages. 



Balbiani believes that the segment of chelicerce is formed 

 later than the segments immediately behind it. He further 

 concludes, from the fact that this segment is cut off from the 

 procephalic portion in front, that it is really part of the 



