1 40 PANTOPODA. 



the development of the embryo. Hoek afterwards tried to complete 

 his account by means of observations made on living specimens 

 (Pdllene, No. 7). Morgan next investigated the cleavage and 

 formation of the germ-layers in these animals (Nos. 10 and 11), 

 and in a more recent work (No. 12) he gives a detailed description 

 of these processes in several Pantopodans. We shall thus have to 

 rely chiefly on his account of these processes. 



In Pallene, the first line of cleavage divides the egg up into two 

 blastomeres, one of these being large, and the other only about a 

 quarter of its size (Morgan). Each of the two spheres is again 

 divided into two by a cleavage taking place at right angles to the 

 first, so that two micromeres and two macromeres are now formed. 

 The third line of cleavage is perpendicular to the two former lines, 

 and gives rise to four micromeres and four macromeres. This stage 

 is followed by one of eight small and eight large cleavage-spheres. 

 From this point onward the micromeres and the macromeres do not 

 divide at the same rate. At a later stage, sections present an 

 appearance like that given in Fig. 65 A, except that the pole of the 

 micromeres consists of smaller cells .than are there figured. The cells 

 are pyramidal, but their boundaries do not in all cases extend to the 

 centre. We here find an indication of transition to the next im- 

 portant stage. 



Unequal cleavage seems also to occur in the eggs of Nymphon brevicaudatum, 

 which are rich in yolk, for, according to Hoek's figure (Fig. 2, PI. xix., No. 6), 

 one half of the egg at a late stage is composed of smaller cells than the other 

 half. 



The nuclei of the pyramidal cells, together with the surrounding 

 protoplasm, shift to the periphery (Fig. 64 A and B), the boundaries 

 of the blastomeres being retained to a certain extent (dp). To some 

 degree, however, they disappear, this being specially noticeable 

 towards the centre of the egg (A and B). The nuclei are surrounded 

 by areas of protoplasm, which send out processes into the yolk. 

 Since these complexes of protoplasm, increasing in number by 

 division and shifting closer together, yield the blastoderm (Fig. 64 C), 

 a stage like that seen in the Araneae is passed through, i.e., the yolk- 

 mass appears first divided up into pyramids which disintegrate later. 

 According to Hoek's description, this breaking up of the yolk can 

 still be distinguished in later stages after the blastoderm has formed 

 (cf. figure of Nymphon brevicaudatum, No. 6, PI. xix., Fig. 5). In 

 the centre of the egg a cavity appears (Fig. 64 A, fh), which must 

 be regarded as the cleavage-cavity. Its occurrence, however, does 



