The Development of Theridium, 311 



its formation by inrolling of the edge of the somatic rather than by 

 vertical mitosis or delamination. At first the two layers are closely 

 apposed, but they later partially separate to produce the coelom 

 (Coel.j Fig. 55). At no time do yolk globules enter any of the coelo- 

 mic cavities. 



In the cephalic region the mesoblast of the cheliceral segment 

 (Chel. Mes., Fig. 56c) is separated from the more anterior rostral 

 mesoblast {R. Mes.). 



In the abdominal segments (Ah. 1, Ah. 2, Fig. 56a) the mesento- 

 blast is transversely and longitudinally segmented into paired bands, 

 but is still one-layered. Posterior to these segments, within the 

 caudal lobe proper (Fig. 56b), the mesentoblast (Mes. E.) is a con- 

 tinuous layer. On transverse section of the caudal lobe are found at 

 occasional intervals in the midline masses of small branched cells 

 (G. C. ?, Fig. 54, compare also Fig. 56b) ; these are much smaller 

 and stain more deeply than vitellocytes, and may represent either 

 genital cells or early definitive entoblast. 



Vitellocytes are still forming by metamorphosis of cells of the 

 extraembryonic blastoderm. 



0. Stage of Three to Five Ahdominal Segments (73 to 75 Hours). 



Fig. 57, PI. V, shows a stage with three abdominal segments. Fig. 

 58 one with four, and Fig. 59 one with five. The abdomen has 

 increased in length until the caudal lobe meets the head lobe on the 

 dorso-anterior surface of the yolk. The posterior unsegmented por- 

 tion of the abdomen is the caudal lobe of the authors, comparable 

 with the telson of other animals having teloblastic gro^vth. While 

 the abdominal segments still lack limb buds the appendages of the 

 cephalothorax have become short and blunt cylinders directed caudad. 

 The head lobe (Oeph., Fig. 59) shows as yet no particular organ 

 regions, but extending from it backwards along the thorax is a light 

 median line, the ventral sulcus (Sul. v.), which marks the region 

 where the ectoblast is thinnest and from which the mesoblast sacs 

 have withdrawn laterally. 



The first traces of the central nervous system now appear, paired 

 thickenings of the ectoblast mesial from the appendages in that 



