310 Thos. H. Montgomery, 



The ectoblast consists of cells that are mainly columnar, and in 

 some places it is becoming two cell layers deep (Figs. 49b, 50a-c). 



The vitellocytes have become relatively enormous (Vit. C, Figs. 

 48, 49b, 50a-c), and most of them lie in the more superficial portion 

 of the yolk, few having reached its center. For the first time the 

 extraembryonic blastoderm is beginning to proliferate vitellocytes by 

 direct metamorj^hosis of some of its cells (Fig. 49a). The extra- 

 embryonic blastodermic cells are also increasing in number. 



5. Stage of One to Tivo Ahdominal Segmenis. 



Fig. 51, PL IV, is an oblique latero-ventral view of the stage, 60 V^ 

 hours, immediately following that of the protozonites. It shows the 

 pedi palpal segment (Fed.) and the segments of the legs {L. 1-L. 4), 

 all with the first traces of limb buds. Fig. 52, an embryo of ca. 62 

 hours, illustrates a lateral view of the later stage with six pairs of 

 cephalothoracal appendages (the cheliceral segment, Cltel., now sepa- 

 rated from the head lobe, Cepli.), and the appearance of the first 

 abdominal segment (Ah. 1). Then Fig. 53 illustrates a still later 

 stage, where there are two abdominal segments (Ah. 1, Ah. 2) and a 

 trace of a third. On comparing Fig. 52 with Fig. 53 it will be seen 

 that it is the abdominal region that is lengthening most rapidly, 

 which results in the caudal lobe {Gaud.) pushing dorso-cephalad 

 until it nearly meets the head lobe. The nuclei of the extraembryonic 

 region are marked by stippling. 



The stage of Fig. 53 merits a more detailed description. 



The mesoblast of the cephalothorax is now arranged in the form 

 of a series of paired pouches, discontinuous longitudinally and trans- 

 versely ; this segregation is the mechanical cause of the limb buds. In 

 the chelicera {Clid., Fig. 56c, PI. V) and the fourth leg pair (L. 4, 

 Fig. 56a) it is only one layer deep, for in these segments it has devel- 

 oped more slowly than in the others ; but beneath the other thoracal 

 appendages it is two cells deep (Figs. 55, 56d). When this two-layered 

 condition has been reached the layer next the ectoblast may be called 

 somatic mesoblast {8o. Mes., Fig. 55), and the other layer, splanchnic 

 mesoblast (Sp. Mes.). The way in which the splanchnic layer be- 

 comes separated from the somatic is shown in Fig. 56d, indicating 



