364 ARANEINA. 



the part of the blastoderm where two rows of cells become first 

 established 1 . 



The whole region of the blastoderm other than the ventral plate 

 is formed of a single row of flattened epiblast cells. The yolk retains 

 its original constitution. 



By this stage the epiblast and mesoblast are distinctly differen- 

 tiated, and the homologue of the hypoblast is to be sought for in 

 the yolk-cells. The yolk-cells are not however entirely hypoblastic, 

 since they continue for the greater part of the development to give 

 rise to fresh cells which join the mesoblast. 



The Spider's blastoderm now resembles that of an Insect (except 

 for the amnion) after the establishment of the mesoblast, and the 

 mode of origin of the mesoblast in both groups is very similar, in that 

 the longitudinal ridge-like thickening of the 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 somewhat 

 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 chelicene 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 origin- 

 ally 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 continuous 

 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 ambulatory limbs : the chelicerse, 

 like the segment to which they belong, lag behind in development. 

 The limbs appear as small protuberances at the borders of their seg- 

 ments. 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 procephalic 

 and caudal lobes being only separated by a very narrow space, the 

 undeveloped dorsal region of the embryo. This is shewn in longitu- 

 dinal section in fig. 204. In this condition the embryo may be spoken 



1 Various views have been put forward by Claparede and Balbiani about the position 

 and significance of the primitive cumulus. For a discussion of which i-ide self, No. 440. 



