CHAPTER V 



EMBRYONIC ENVELOPES, DORSAL ORGANS, 

 AND BLASTOKINESIS 



THE EMBRYONIC ENVELOPES 



During the course of early embryonic development there are formed 

 in most insects two protective envelopes, or membranes: an outer serosa 

 and an inner amnion, the former a derivative of the extraembryonic 

 primary epithelium (blastoderm), the latter developing from the margin 

 of the embryonic rudiment. The steps in the development of these 

 membranes in the most typical case is represented in the accompanying 

 figures. In initiating the development of amnion and serosa an amnio- 

 serosal fold appears in the periphery of the germ band (Fig. SlB,amf), 

 the inner part of the fold forming the amnion, the outer part (extra- 

 embryonic primary epithelium) forming the serosa. With the extension 

 of these membranes the lips close on each other so that finally the serosa 

 lies at the surface immediately under the vitelline membrane and covers 

 the entire egg while the amnion covers the ventral face of the developing 

 embryo (Fig. SIC). At a later period just before the reversal of the 

 embryo, when their development has well advanced, the serosa and 

 amnion become fused in the head region (Fig. SlD,am.ser). A rent 

 appears in this fused area, the margins of the rent, however, remaining 

 fused so that the edges of the amnion and serosa adhere to each other 

 (Fig. SlE,amf). The membranes then contract (Figs. S1E,F) toward 

 the dorsal side of the embryo where the amnion {am), turning inside out 

 in the process, forms a provisional dorsal wall and the serosa a tubular 

 so-called "secondary dorsal organ." Both these structures thereupon 

 undergo degeneration (Fig. 32, do). Another structure, known as the 

 "primary dorsal organ," which develops early behind the head in 

 the Collembola and some other insects, will be referred to later. The 

 description given above of the position and origin of the germ band is 

 that of a generalized type which is subject to many modifications. These 

 variations may be traced back to the shape of the egg, the amount and 

 distribution of the food yolk, and the manner in which the embryo 

 grows. 



Certain features in embryological development occur to which terms 

 have been applied that may here be defined. When the yolk penetrates 

 between the amnion and the serosa, the germ band is said to be immersed, 



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