THE FURTHER DEVELOPMENT OF THE EMBRYO. 



145 



The order of appearance of the limbs varies in the different forms. 



In Pallene, according to Morgan, the first to develop is the most 



anterior pair; these limbs lie near the mouth and are chelate, but 



their first appearance has not been observed with certainty. The next 



pair to arise is the fourth, and, in the gap which naturally occurs 



between these limbs, two pairs 



of ganglia are visible, those of 



the second and third pairs of 



limbs (Figs. 66 and 67 A). 



The fourth pair of limbs is 



followed by the fifth and sixth. 



The third pair develops later, 



but the second pair is alto- 

 gether wanting in Pallene, and 



the seventh pair, like the 



third, appears a short time 



before the embryo leaves the 



egg-envelope. Pallene is thus 



seen to possess, as an embryo, 



all the limbs of the adult. 



In most other Pantopoda, 



however, this is not the case, 



only three pairs of limbs being 

 usually developed within the 



egg-envelope. Nymphon brevicaudatum resembles Pallene in possess- 

 ing all the limbs of the adult at the time of hatching (Hoek). 



While the limbs are appearing and gradually developing, the rudi- 

 ment of the nervous system also undergoes further differentiation. 

 Five pairs of large ganglia can be distinctly made out (Fig. 67 A). 

 They belong to the segments carrying the second to the sixth limbs. 

 It would be interesting to discover the relations of the ganglia which 

 innervate the first pair of limbs to the supra-oesophageal ganglion, 

 i.e., whether they represent a post-oral pair of ganglia fused with 

 the supra-oesophageal ganglion. The two anterior of these five pairs 

 of ganglia approximate closely to each other later (Fig. 72 B), and 

 in the adult these two ganglia, belonging to the second and third 

 limbs, are united. The ganglia of the first three pairs of ambulatory 

 limbs (Fig. 67 A), which appear early, are followed at a much later 

 stage by those of the fourth pair (the seventh pair of limbs) and 

 the abdominal ganglia. 



In each of the ectodermal thickenings which represent the rudi- 



Fig. 6C— Superficial aspect of an egg of Pallene, 

 showing the anterior part of the embryonic 

 rudiment (after Morgan), g, rudiments of the 

 supra-oesophageal ganglia ; gji-gjy, ventral 

 ganglia belonging to the segment carrying the 

 second, third, and fourth pairs of limbs ; m, 

 mouth ; I, first limb ; IV, rudiment of the 

 fourth limb. 



