208 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



Unlike the heart, the aorta is not formed from cardioblasts but 

 directly from the median or inner wall of the antennal coelomic sac which 

 has extended back into the neck region. The lateral or outer wall of the 

 antennal sacs form the peritoneal wall of the stomatagastric ganglia. As 

 soon as the secondary dorsal organ has been absorbed in the yolk, the 

 continuity of the heart and aorta is established. The subesophageal 

 body is lacking in Forficula. 



According to Heymons (1895), the mid-gut epithelium is formed from 

 ectodermal cells arising from the blind ends of stomodaeum and procto- 

 daeum. These cells form a single-layered ribbon with forked end from 

 each of the invaginations. The ribbons elongate until they meet, then 

 widen to envelop the yolk. Strindberg (1910) interpreted them as 

 entodermal. 



Before hatching, the amnion and serosa rupture and are absorbed in 

 the yolk, as with the Odonata. 



HEMIMERINA 



Hemimerus talpoides 



Hemimerus talpoides, an external parasite upon an African giant rat 

 (Cricetomys) , is remarkable in that it exhibits an intra-uterine develop- 

 ment, the embryo undergoing its complete development in the ovary 

 nourished by the aid of a placenta-like structure. The embryonic 

 growth and the structure of the reproductive organs have been described 

 by Heymons (1912); the anatomy, by Jordan (1909). 



The egg follicle of the female, as in other insects, at first forms an 

 epithelium of a single layer of cells. Later, as development proceeds, the 

 nuclei arrange themselves in an irregular fashion in the vicinity of the 

 egg, forming the follicular placenta, by means of which the embryo is 

 nourished. The cells of the follicular placenta in the immediate vicinity 

 of the nurse cell undergo an especially active proliferation whereby the 

 latter is soon completely enveloped. The nurse cell, which lies at the 

 apical end of the egg follicle, then becomes functionless. The growing 

 embryo from this time on derives some nourishment from the placenta 

 follicle but chiefly from the mass of proliferating placenta cells that sur- 

 round the degenerated nurse cell, as well as from a similar placenta cell 

 mass at the posterior end of the egg follicle. 



Instead of yolk, the shell-less egg contains a fatty substance which 

 later becomes alveolar. In spite of a lack of yolk the cleavage is not total, 

 the cleavage nuclei migrating to the surface especially toward the poste- 

 rior part of the egg where the germ disk is probably formed. Nuclei that 

 remain behind in the interior will form the trophocytes (entoderm cells). 

 The germ band, at first composed of a single layer of cells, in the process of 



