EMBRYONIC DEVELOPMENT. 



95 



Claparede's account (No. 77) is still the most complete.* According 

 to this writer, in Tetranychus telarius, the nucleus, surrounded by 

 formative protoplasm, rises to the surface of the yolk (Fig. 49 A) 

 and soon divides. Eepeated division (Fig. 49 B) gives rise to a 

 large number of nuclei, each surrounded by an area of protoplasm. 

 The nuclei remain lying at the surface of the egg, and by increasing- 

 still further in number, they, with the protoplasm around them, give 

 rise to the blastoderm (Fig. 49 G). 



pr 



Fig. 50.— Embryonic development and formation of the first larval integument in Myobia 

 musculi (after Claparede, from Balfour's Text-hook). In D, the egg-integument has split, 

 and the embryo, surrounded by the first larval integument, is in the act of leaving the egg. 

 ch, chelicerae; pd, pedipalps; pHA the first three pairs of limbs; pr, proboscis (which 

 has arisen through the fusion of the chelicerae and the pedipalps) ; sL-s*, four post-oral 

 segments. The yolk is represented by the darker area. 



According to Robin and Megnin (No. 104), total cleavage occurs in the 

 eggs of the Sarcoptidac. The egg, while still in the oviduct, was seen to hreak 

 up into four cleavage-spheres. This, if correct, reminds us of the condition 

 described in the Araneid egg, which, however, does not there lead to the 

 complete division of the egg into cleavage-spheres. Total cleavage is said to 

 he undergone also by the egg of Cliclifer (p. 2S), and the same has been main- 

 tained, at least at a later stage, of the eggs of the Opiliones also (p. 32). 



The blastoderm was examined in a number of Acarina, and always 

 consisted of a thin single layer of cells enclosing the yolk. As it 

 develops further, thickenings take place at points corresponding to 

 the future ventral surface, especially in the cephalic and caudal 

 i-egions. The germ-band thus arises here (Fig. 50 B) in the same 

 way as in other Arachnids. At first it is an ecpually thickened 



* [Henking's account is perhaps more correct.— Ed.] 



