208 Dr. J. Barrois on the 



partition, like those which keep the digestive tube of the 

 Annelids in place. 



We have already seen that at this epoch these four zonites 

 had arrived at their maximum development, and played the 

 principal part in the envelopment of the vitelline vesicle by 

 the tergal plate ; at the period when this envelopment is 

 completed, the segments of the postabdomen still occupy 

 only a small space in the posterior part, and the abdomen is 

 almost entirely formed by the first four preabdominals, the 

 diaphragmatic partitions of which divide the vitellus into four 

 large digitate masses, recognized and well figured by Herold 

 and especially by Claparede, but of which they did not under- 

 stand the signification. The abdomen of nearly all spiders is 

 thus almost entirely composed of these four extraordinarily 

 developed segments, which at last shows us their signification. 



At the same time that these various phenomena take place, 

 the space left free (fig. 2, bv) by the tergal plate is covered 

 up by the sternal plates. It is at this period that the limb- 

 rudiments of the first four abdominal segments disappear, and 

 that the boundary between the dorsal and ventral halves of 

 the body seems to disappear, to give place to an abdomen of 

 globular form, in which the limit between the sternals and the 

 tergals is no longer precisely indicated. We see that at the 

 period of this disappearance the ventral pads (in consequence 

 of the growth of the tergal plate) occupy a very different posi- 

 tion (bv, fig. 2) than was assigned to them by Claparede at 

 the same epoch : the distinction between tergal and sternal 

 arches persists to the last irf the Spiders ; but finally, as we 

 have seen, there is no proportion between the two pieces, the 

 tergal pieces having grown out of all proportion, and the 

 sternals forming only an insignificant part. The complete 

 disappearance of the lateral pads is caused by a displacement 

 of the last vestiges of the germinative bands. We have seen 

 that these last continued hitherto to form a slight thickening 

 on the edges of the tergal plate ; at the period when the 

 sternals increase to cover up the portion left free by the tergal 

 plate (fig. 2, bv), this thickening quits the lateral region and 

 unites with that of the opposite side to form a compact meso- 

 dermic mass of oval form, upon the median line and imme- 

 diately within the space covered up by the sternals. From 

 this thickening will be formed later on the whole of the 

 straight abdominal part of the digestive tube, the excretory 

 organ, the spinning-glands, and the genital organs. The 

 spinnerets themselves originate at this period, in the form of 

 pretty large elevations of the skin, situated at the posterior 

 limit of the sternal plates : at first they only appear in 



