206 Dr. J. Barrois on the 



This fact teaches us to compare this stage with the fossil 

 forms of the Merostomata, which so often resemble passage- 

 types between the Scorpions and the Xiphosura. Among the 

 latter there is even a form {Hemiaspis limuloides) which 

 seems to me to recall in a striking manner the stage which 

 I have indicated in the Spiders : in it the body is in like 

 manner separated into two divisions, of which the posterior 

 contains ten segments, which are divided in the same way 

 into six preabdominal and four postabdominal, the last of 

 which is elongated into a style ; it may also be noted that 

 the first four preabdominals in this species present traces 

 of an organization somewhat superior to that of the two 

 following ones. This predominance of the first four pre- 

 abdominals moreover appears to me to be a very constant 

 fact, not only among the Spiders, but in the whole group 

 Arachnida. 



MetschnikofF has already shown that in the Scorpions they 

 appear at the same time and before all the others, and that 

 the formation of the ventral ganglia in their interior also 

 greatly precedes that of all the others. Unfortunately we do 

 not possess, with regard to the Limidi, data sufficiently com- 

 plete to enable us to judge whether this is the case in the 

 Xiphosura. Fig. 1 shows that at this period these segments 

 in the spider occupy a considerable space and form more than 

 half the tergal plate. 



Besides the embryo proper (fig. 1), we have still to consider 

 in this stage the vitelline part (figs. 2, 3), bounded all round 

 by the nervous bands and the sternal plates (fig. 3) . This 

 now represents only an annex of the embryo, and on all 

 accounts merits the name of vitelline vesicle ; in fact, as in 

 the fishes, it forms a sac surrounded by a delicate blastoderm, 

 projecting from the ventral surface ; it is bounded on all sides 

 by the embryonic parts, and, in like manner, owes its origin 

 to a displacement towards the ventral part of a nutritive 

 vitellus too abundant to be entirely contained within the 

 embryo. Thus this hernia of the nutritive vitellus through 

 the sternal fissure, regarded by Claparede as constituting a 

 mode of reversal (retournement) of a special nature differen- 

 tiating the development of the Spiders from that of the other 

 Arthropods, is due, according to me, solely to the presence in 

 the Spiders of a vitelline vesicle exactly like that of fishes : it 

 is, I believe, the first instance to which attention has been 

 called in the Invertebrata *. 



Leaving out of consideration this presence of the vitelline 



* Except perhaps in the Salpes. 



