THE BODY-CAVITY AND THE BLOOD- VASCULAR SYSTEM. 201 



towards the dorsal median line, while the greater part of the lateral 

 portion is withdrawn into the rudiments of the limbs (Fig. 104, 

 v. Kennel, Sedgwick). 



Even before this separation has commenced, while the primitive segments 

 still retain their sac-like shape, the antero-dorsal portion of each grows forward 

 over a part of the preceding primitive segment, and thus extends into the 

 preceding body-segment. This explains the fact that in transverse sections we 

 not only see the segmental cavity of the segment through which the section 

 passes, but also a portion of that belonging to the next segment, and that this 

 latter lies above the ventral portion of the segmental cavity of the preceding 

 segment. 



The lateral portions of the primitive segments yield the nephridia, 

 and the dprso- median the genital glands in the segments which 

 contain these organs ; in the other segments these portions disappear, 

 their cell-elements being used in the formation of the blood vascular 

 system and the musculature, and for the further development of the 

 pseudocoele, which now comes under consideration. 



The Body-cavity and the Blood-vascular System. 



Even before the division of the primitive segments into two 

 portions, the ectoderm had separated from the entoderm with which 

 it was until then in close contiguity, thus giving rise to a free space 

 dorsally and ventrally to the intestine. These spaces are the first 

 indication of the body-cavity of the adult (Fig. 100 A and B, Hi), 

 and into them the mesoderm-cells which become detached from the 

 primitive segments wander. As these cells become applied to the 

 entoderm and ectoderm, the cavity which is at first bounded merely 

 by these two germ-layers, and is therefore to be regarded as the 

 primary body-cavity, becomes lined with mesodermal elements 

 (Fig. 101 A, Hi). These space?, in consequence of their origin, 

 are not segmented, but the other and lateral portions of the future 

 body-cavity, which arise by separation of the cell-elements in the 

 inner thickened somatic wall of the lateral portions of the primitive 

 segments, exhibit a segmental arrangement (Fig. 101 A, l.lh). 

 These cavities, at first distinct from one another, fuse together later, 

 and give rise to the two spaces, the lateral sinus of Sedgwick, which 

 later run continuously through the body, and in which the nerve- 

 strands lie in the adult. Another space on each side of the body 

 agreeing in origin with these latter spaces, develops still more 

 peripherally in the limb-rudiments and surrounds the nephridia 

 (Figs. 101 and 102, p.lli). This last part of the body-cavity, which 

 may best be described as the pedal body-cavity, unites later in some 



