202 Dr. J. Barrois on the 



rapidly inwards, is destined to form the oesophagus, the suck- 

 ing-stomach, and the portion of the intestine which immedi- 

 ately follows — in one word, all the median tubular portion of 

 the digestive canal contained in the thorax : it is quite com- 

 pleted at the commencement of the third period, in the young- 

 spider rolled up in the egg. 



As this invagination penetrates inwards it gives rise to con- 

 comitant modifications, which are of great importance in 

 understanding the structure of the thorax. If we make a 

 transverse section in the middle of the thorax of a spider, we 

 see that the musculature is constituted as follows (see fig. 6) : — 

 1, a central part formed by three muscular bundles (m) 

 attached to the oesophagus ; the first, which is unpaired, is 

 vertical, and is attached to the vertex of the body ; the other 

 two, paired, form a horizontal plane, and, covering the ventral 

 ganglionic masses, pass to attach themselves to the bases of 

 the Tegs ; 2, of a peripheral part {711') composed of fibres which 

 skirt the wall, and which constantly go from the bases of the 

 feet to the vertex of the body. This second part of the mus- 

 culature is of comparatively late formation 5 it does not make 

 its appearance until rather late, and only acquires its full 

 growth during the postembryonic development : the first, on the 

 contrary, appears early and proceeds pari passu with the invagi- 

 nation of the anterior part of the digestive tube. It would seem, 

 in fact, that this invagination carries with it in its movement 

 towards the posterior part (see figs. 4, 5) the peripheral por- 

 tions of the germinative bands, in such a way as to form three 

 cellular laminae — a median one detached from the cephalic 

 portion, the other two lateral ones from the two germinative 

 bands. These three lamina? represent the three muscular 

 planes marked m in fig. 6 : the first, which directly surrounds 

 the oesophagus (fig. 4, m), represents a longitudinal partition 

 which will divide the cavity of the thorax into two parts ; the 

 two others, two transverse partitions which will soon unite 

 into a single plane directly covering the ventral ganglia. Soon 

 after the last stages of the embryonic band the cephalic portion 

 is rounded behind, and, as Claparede has already stated, sepa- 

 rated from the vitellus by a deep emargination ; but after the 

 period when the three cellular plates (m, fig. 4) begin to form, 

 we see it. produced into an elongated point, which is already 

 figured by Claparede, and represents the vertical partition 

 (m, fig. 6) which gradually elongates simultaneously with the 

 oesophagus. 



Figs. 4 and 5 show the three planes at the moment when 

 they begin to detach themselves from the germinative bands. 

 In proportion as the invagination of the oesophagus progresses 



