56 THE CHANGES FROM STAGE A TO F, 



whether the external transverse muscles of the body wall are 

 ectodermal or mesodermal. 



2. The lateral thickenings, which give rise, amongst 

 other things, to the nervous system, contain during Stages a 

 and B a single layer of long oval nuclei (Plate VI, fig. 5 b). 

 They are at first confined to the ectoderm immediately over- 

 lying the mesoblastic somites, and constitute on each side a 

 continuous band extending from the pre-oral region, where 

 they are more ventrally placed and continuous with each other 

 across the middle line, to the primitive sti'eak behind. They 

 diverge from one another as they pass backwards, and end in 

 the ectoderm of the primitive streak. During Stages a 

 and B they contain a single layer of oval nuclei, which rapidly 

 increase in number, and become arranged in Stage c in parts 

 in more than one layer. The latter fact is especially con- 

 spicuous in the pre-oral portions, i. e. in the portions which 

 will give rise to the cerebral ganglia (Plate VI, fig. 6 a). 



In Stage d the lateral thickenings have increased consider- 

 ably in extent, occupying the whole of the sides of the body, 

 and encroaching on the middle region of the body, where in 

 the previous stages they were widely apart, somewhat on the 

 ventral surface (Plate VI, figs. 9, 11), so that the relation to 

 the somites mentioned above (p. 55) is lost. The increase in 

 the number of nuclei is now more marked, and is found to 

 concern chiefly two regions : (a) nearly the whole of the pre- 

 oral parts of the lateral thickenings (Plate VI, fig. 14) ; {b) 

 the ventral (inner) portions only in the circumoral and postoral 

 regions (Plate VI, figs. 9, 13). The increase has for its re- 

 sults, as may be seen by reference to the figures, the production 

 of some roundish nuclei lying internally to the oval nuclei. These 

 round internally placed nuclei are much more numerous in 

 Stage E, and eventually give rise to the whole of the cerebral 

 ganglia and ventral nerve-cords. They remain up to the 

 close of Stage e in close connection with the external layer of 

 oval elements, though in some sections there are, in the post- 

 oral region, indications of a commencing separation between 

 them (Plate VII, fig. 23 a — d). The manner of separation of 



