246 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



tenth abdominal segment are very large. As they develop, they move 

 medially and eventually open into the ejaculatory duct. The male acces- 

 sory glands arise as invaginations of the walls of these ampullae. 



About a day after blastokinesis the embryo secretes a cuticular mem- 

 brane all around its body which is shed just after emergence. Hatching 

 and intermediate molting are dependent on humidity but not influenced 

 by light. Hatching in Locusta migratoria occurs 7 days after blastokine- 

 sis, or 13 days after egg deposition. 



The Differential Locust {Melanoplus differentialis) 



As described by Nelsen (1934), the embryonic rudiment of Melanoplus 

 differentialis is at first a short, pyriform, posteriorly placed germ disk; 

 then it becomes short T-shape, then progressively longer as segments 

 are differentiated from the cephalic toward the posterior end. A blasto- 

 poric groove forms in the germ disk, and soon the inner layer begins to 

 form, not only by invagination and delamination but also during these 

 processes by division of the inward-moving cells. A tubular invagination 

 does not form. The mid-gut rudiment is derived from the ends of the 

 inner layer associated with the blind ends of stomodaeum and procto- 

 daeum, although Nelsen does not deny the possibihty of ectodermal 

 material's migrating inward w^ith the entoderm. The mid-gut epithelium 

 thus develops from anterior and posterior rudiments. Aside from the 

 material that is segregated for the formation of mid-gut epithelium, 

 the inner layer constitutes the mesoderm. Mesoderm material in the 

 head and thoracic regions begins to differentiate before the abdominal 

 region has attained its caudal limits of growth. Mesodermal segmenta- 

 tion corresponds to ectodermal segmentation, with paired coelomic 

 cavities appearing in the appendage-bearing segments. 



The germ cells are segregated from the lateral margins of the ectoderm 

 of the abdominal region at the time the segmentation of the latter begins. 

 They ultimately become separated from the ectoderm cells where these 

 join the amnion and migrate in a passive manner on to the coelomic sacs 

 where they become associated with the inner walls of the sacs. When the 

 coelomic sacs coalesce with each other, the germ cells and the splanchnic 

 wall mesoderm cells form two continuous cell strands from the first to the 

 eighth abdominal segments, inclusive, although some cells may be located 

 posterior to the eighth segment. 



Recently Stuart (1935) working with the same insect arrived at a 

 different conclusion regarding the origin of the mid-gut epithelium. He 

 says that shortly before hatching, the yolk cells move peripherally to 

 form a temporary lining upon the inner surface of the mesodermic com- 

 ponents of the mid-gut. About the time the insect hatches, each yolk- 

 cell nucleus divides into a dozen or more smaller nuclei which Stuart 



