VI 



THE AMNION AND THE CEPHALO-DORSAL BODY 



i. The Amnion 



The cells destined to form the amnion, as described on p. 44, 

 constitute the marginal portions of the ventral plate, and cover 

 the dorso-lateral regions of the egg, bounding the median dorsal 

 strip laterally (Figs. 19 and 24A, Am). At first the amnion-form- 

 ing cells are not sharply demarcated from the remainder of the 

 ventral plate, and seem to intergrade with the cells of the latter, 

 but as the differentiation of the germ layers progresses the amnion 

 cells also become differentiated, appearing more or less shortened 

 or flattened as contrasted with the long prismatic form of the 

 cells of the embryonic portion of the ventral plate. As soon as the 

 amnion cells become distinguishable as such, it becomes evident 

 that the amnion cells at the anterior end of the egg differ some- 

 what from those of the other regions. Over the entire cephalic 

 end of the egg, the amnion cells are rounded in form, closely ar- 

 ranged and relatively numerous, in sections resembling a string 

 of beads (Fig. 24A and 26A, Am). The characteristic transpar- 

 ency of the inner ends of the amnion cells, noted by Petrunke- 

 witsch (1903) is especially evident. Caudad of the cephalic 

 region the amnion cells become relatively fewer and their form 

 also becomes more flattened (Figs. 2oA and B and 21, Am). 

 These differences are, however, merely temporary and disappear 

 as the amnion increases in extent. Prior to Stage V the amnion 

 therefore consists of two longitudinal bands of epithelial cells, 

 separated by the median dorsal strip and joining the ventral plate 

 laterad. These amniotic bands are widest at their anterior ends, 

 which cover the cephalic end of the egg, and their component 

 cells are here more numerous and less flattened than elsewhere, 

 as just mentioned. At Stage V the two bands begin gradually to 

 widen, the cells of their inner margins creeping up over the dorsal 



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