DEVELOPMENT OF THE EXTERNAL FORM. 7 



chelicerae), a large segment (that of the pedipalps), another in the 

 act of forming (that of the first pair of limbs), and, finally, a large 

 caudal region. By the separation of other segments from the latter, 

 the number of body-somites is increased. The most anterior of 

 these, with the exception of the primary trunk-segment, seem the 

 most developed. They become less distinct as the caudal region 

 is approached. A furrow appearing in the middle line (neural 

 groove), which is concerned with the formation of the ventral 

 chain of ganglia, and is in no way connected with the median 

 longitudinal furrow already mentioned, divides the germ-disc into 

 two symmetrical halves (Fig. 4 B). The similarity between this 

 embryonic rudiment and the germ-band of other Arthropoda and of 

 Periporitis is now very pronounced. The germ-band lies upon the 

 yolk, with its ventral surface turned outwards. In the region 

 occupied by the germ-band, which includes the greater part of the 

 germ-disc, the latter appears much thickened (Fig. 3 B) : the germ- 

 layers spread over the yolk beyond the germ-band, but there appear 

 much less developed. They gradually grow round the whole of the 

 yolk, which thus comes to lie inside the embryo. 



The circumcrescence of the yolk by the cell-layers, which have long been 

 differentiated as germ-layers, cannot be regarded as gastrulation, as was thought 

 by Balfour. Neither does the blastopore lie on the dorsal surface, but is rather 

 to be sought on the ventral surface, in the middle of the germ-disc (p. 2). 



When about ten segments have appeared (Metschnikoff), or 

 perhaps earlier (Laurie), the limbs become apparent. They arise 

 as outgrowths of the segments on each side of the middle line 

 (Figs. 4 G and 6), and are hollow and truncated, the primitive 

 mesoblastic segments which have already attained development lying 

 for the most part within them, a feature which we shall find exactly 

 reproduced, not only in the Araneae, but in Peripatus and in the 

 lower Insects. The development of the limbs also takes place from 

 before backward, but the chelicerae are remarkably late in developing 

 (Fig. 4 C). When the pedipalps are already large, the chelicerae are 

 no more than small prominences (Fig. 6). This must be explained 

 by their smaller size in the adult. The chelicerae as well as the 

 pedipalps are without doubt post-oral in position, for the mouth first 

 appears quite anteriorly between the cephalic lobes (Fig. 6, m). In 

 front of the mouth, an unpaired structure, the upper lip (or rostrum), 

 appears later (Fig. 7 B). The rudiments of the four pairs of limbs 

 which follow greatly resemble the chelicerae and pedipalps both in 

 form and in position (Figs. 5 and 8). The series of thoracic limbs 



