444 Avery E. Lambert. 



variously as representing the anlage of the endoderm, and as being 

 the cells from which the blood corpuscles are derived. That they are 

 not blood cells is clearly evident from the fact that after the forma- 

 tion of the heart by the union of the two walls of the dorsum, no cells 

 of this sort are to be found in its lumen. At the same time they are 

 abundant at the posterior end of the stomodseum, and in the region 

 of the proctodeum and stercoral pocket, as well as being noticeably 

 abundant in the region where the genital organs of the spider are 

 formed. 



Stage XI. Fig. 32. — This is the earliest period in the development 

 of the spider in which, the various parts of the brain have assumed 

 approximately the position they will occupy throughout the life of 

 the adult. In external form the young spider closely resembles the 

 adult, although no pigment has appeared in either the eyes or the 

 skin. 



The internal organization of the embryo is, however, very incom- 

 plete. The lung-books have arisen ; the spinning glands have formed 

 and are nearly functional. But the mid-gut is lacking (Fig. 34, 

 oes. ) , the abdomen being filled with a mass of modified yolk which is 

 traversed by rudimentary vessels, and by septa?, springing from the 

 walls of the heart. 



The anterior median eyes have advanced considerably in their 

 development. The corneal layer of the eyes is directly continuous 

 with the ectoderm which covers the outer portion of the head ; the 

 infolded portion of the eyes bears the retina and, turning again, pos- 

 teriorly, forms a third, or post-retinal layer. (Fig. 33, m. e.) 



The posterior median eyes and the lateral eyes are also consider- 

 ably advanced. Following Bertkau (4) these may be called acces- 

 sory eyes (nebenaugen) , the anterior median eyes being called by 

 this author the principal eyes (Jimiptaugen). 



The accessory eyes do not arise in relation to any special infolding 

 after the manner of the principal eyes. They first appear as thick- 

 uings of the ectoderm in the region of the optic plate, and are situated 

 a little in advance of the bases of the chelicerse. As the optic fold 

 advances the accessory eyes become located on its outer portion, the 

 thickened area, in the case of each eye, forming the bottom of a 



