THE EMBRYOLOGY OF INSECTS AND ARACHNIDS. 21 



It will be remembered that the amnion, in the late stages of development of the 

 grasshopper, is absorbed, with the exception of a portion attached to the dorsal 

 extremities of the body walls of the embryo. 



The outer membrane or serosa, which is equivalent to the original blastoderm 

 minus that portion which forms the amnion, is also absorbed. But before this 

 absorption occurs a layer of yolk cells is formed beneath it. This layer becomes 

 specially thickened on the dorsal surface of the yolk and forms a dorsal organ. A 

 similar, dorsal organ has been described for other insects. It is, however, totally 

 different from the dorsal organ described by Brant for Libellula. It will be remem- 

 bered that no corresponding structure occurs in the development of Thyridopteryx. 

 The dorsal organ* becomes concentrated at one point on the dorsal surface. Its cells 

 multiply and migrate into the yolk which they digest or prepare for digestion. The 

 body walls are then closed dorsally ; the dorsal organ for a time remains as a tube 

 on the surface of the body. A similar behavior of the dorsal organ has been noticed 

 in other insects. In the latest stages which I studied, it will be remembered that 

 in the grasshopper the serosa or blastoderm persisted on the surface of the yolk ; 

 and there appeared to be nothing present or in process of formation which corres- 

 ponded to a dorsal organ. 



According to Korotneff, the mesoderm is distinctly divided into splanchnic and 

 somatic portions The former enclosing in the body cavity the yolk which is 

 arranged in pyramids with yolk cells at their bases. No marked separation of 

 mesodermic layers was observed in the grasshopper. 



Migratory mesoderm or mesenchyme cells arise from the mesoderm. These 

 resemble similar cells described in Thyridopteryx. Blood cells arise from mesoderm 

 on the median ventral surface. Their histological structure is not described by 

 Korotneff. They may, however, be like the cells described as blood cells in the 

 advanced grasshopper embryo. 



The heart is formed from folds of the somatic layer of mesoderm growing 

 together over the dorsal surface somewhat in the manner described by Dohrn. (1) The 

 nervous system arises according to Korotueff as a single string which becomes sep- 



(11 Notzien zur Kenntniss d. Insectenentwicklung. 



