144: PANTOPODA. 



Arachnida over a limited area of the blastoderm. Morgan states very definitely 

 that this budding-off of an inner layer of cells or multipolar delamination takes 

 place in Pallene slowly, while the outer layer of cells is growing round the yolk ; 

 this we might perhaps refer to an ingrowth of cells with a circumcrescence of the 

 yolk, and compare this process with the corresponding one in the Scorpion. It 

 is advisable to direct the attention of future observers to this point. When the 

 depression of the blastoderm described above appears, the entoderm, according 

 to Morgan, is already formed ; the depression could not, therefore, be compared 

 with the blastopore, although in other respects such a comparison is suggested, 

 all the more so that Morgan thinks that the mesoderm arises round this 

 depression. The fact that other processes in the Pantopoda resemble those in 

 the Arachnida is proved by the formation of a germ-band which, however, is 

 much degenerated, but at the same time shows a certain resemblance to that of 

 the Arachnida. 



Eggs rich in yolk no doubt represent the more primitive condition in the 

 Pantopoda, and the formation of a blastoderm (of the usual Arthropodan consti- 

 tution) and of a germ-band must also be regarded as primitive. The reduction 

 of the yolk probably had a great influence on the ontogenetic processes, which 

 thus attained the condition in which they are now found (p. 154). 



2. The Further Development of the Embryo. 



Our knowledge of the development of the embryo and the origin 

 of its organs is still very incomplete. The following accounts refer 

 chiefly to Pallene, Avhich was made the subject of careful investigation 

 by Morgan. We must, however, point out that Pallene, unlike 

 other Pantopoda, remains within the egg almost up to the time when 

 the adult form is reached (pp. 148 and 153). 



When the invagination already mentioned has appeared on the 

 thickened side of the blastoderm, other thickenings of the surface 

 take place. Two of these are oval in form (Fig. 66, g), and lie in 

 front of the triangular depression (?»). These represent the rudiments 

 of the supra-oesophageal ganglion. Extending posteriorly from the 

 invagination are two rows of thickenings ; these are the rudiment of 

 the ventral chain of ganglia {gn-givj ', laterally to these the first 

 indications of the limbs appear as distinct thickenings (Fig. 67, A). 

 These rudiments, taken as a whole, form a band on the ventral 

 surface of the egg, narrow anteriorly, but broader posteriorly, which 

 may with safety be compared with the germ-band of other Arthro- 

 pods. As the yolk-mass is not very large, the germ-band covers a 

 great part of the egg. As the embryo develops further, it extends 

 laterally, covering a still larger part, so that it can no longer be 

 designated as a distinct germ-band, but rather as the embryonic 

 rudiment surrounding the egg. During this process the embryo has 

 also grown somewhat longer (Fig. 67 A). 



