440 AR AN KIN A. 



mesoblast shewn in fig. 199 is probably the homologue of the 

 mesoblastic groove of the Insects' blastoderm. 



The ventral plate continues to grow rapidly, and at a some- 

 what later stage (fig. 200 B) there are six segments interposed 

 between the procephalic and caudal lobes. The two anterior of 

 these (ch and pd\ especially the foremost, are less distinct than 

 the remainder ; and it is probable that both of them, and in any 

 case the anterior one, are formed later than the three segments 

 following. These two segments are the segments of the chelicerae 

 and pedipalpi. The four segments following belong to the four 

 pairs of ambulatory legs. The segments form raised transverse 

 bands separated by transverse grooves. There is at this stage a 

 faintly marked groove extending along the median line of the 

 ventral plate. This groove is mainly caused by the originally 

 single mesoblastic plate having become divided throughout the 

 whole region of the ventral plate, except possibly the procephalic 

 lobes, into two bands, one on each side of the middle line (fig. 

 203 B). 



The segments continue to increase in number by the con- 

 tinuous addition of fresh segments between the one last formed 

 and the caudal lobe. By the stage with nine segments the first 

 rudiments of the limbs make their appearance. The first 

 rudiments to appear are those of the pedipalpi and four, ambu- 

 latory limbs : the chelicerae, like the segment to which they 

 belong, lag behind in development. The limbs appear as small 

 protuberances at the borders of their segments. By the stage 

 when they are formed the procephalic region has become 

 bilobed, and the two lobes of which it is composed are separated 

 by a shallow groove. 



By a continuous elongation the ventral plate comes to form 

 a nearly complete equatorial ring round the ovum, the pro- 

 cephalic and caudal lobes being only separated by a very narrow 

 space, the undeveloped dorsal region of the embryo. This is 

 shewn in longitudinal section in fig. 204. In this condition the 

 embryo may be spoken of as having a dorsal flexure. By the 

 time that this stage is reached (fig. 200 C) the full number of 

 segments and appendages has become established. There are 

 in all sixteen segments (including the caudal lobe). The first 

 six of these bear the permanent appendages of the adult ; the 



