THE LATER EMBRYONIC DEVELOPMENT. 65 



Opposite the neuropore, and corresponding to the marked 

 thickening in the ventral wall of the neural tube already 

 described, there is a distinct bending of the notochord (Fig. 33), 

 traces of which persist even in the adult animal. 



The histological development of the notochord presents some 

 features of interest. The interdigitation of the cells of the two 

 sides, the commencement of which is shown in Fig. 29, proceeds 

 rapidly ; and, at the time of its separation from the gut, the 

 notochord consists (Fig. 32) of four or five rows of cells, 

 arranged horizontally one above another, each cell extending 

 across the whole of its width. Within the notochordal cells 

 numerous small vacuoles now appear ; these vacuoles are, from 

 the first, most abundant in the two middle rows of cells, and in 

 these they increase greatly in size ; so that in its later stages, as 

 in the adult, the notochord consists of a middle series of cells, 

 enormously distended by vacuoles, and covered on its dorsal and 

 ventral surfaces by rows of smaller and comparatively little 

 modified cells. 



3. The Mesoblastic Somites. 



The mesoblastic ridges, as described above, are a pair of 

 longitudinal folds of the dorso-lateral walls of the archenteron, 

 inclosing slit-like diverticula of the archenteric cavity (Figs. 

 26, 28). By transverse constrictions these ridges become 

 divided into somites, which, though separated from one another 

 by the constrictions, still retain for a time their communi- 

 cations with the archenteron (Figs. 27, 28). 



At the time of hatching, two pairs of these somites are 

 present ; and, as the embryo elongates, other pairs are added in 

 succession from before backwards, the number of pairs of 

 somites present affording a convenient basis for estimating the 

 age of an embryo (Figs. 27, 30, 33). 



The anterior somites, which are the first formed, are also 

 the largest, and the remainder decrease in size towards the 

 hinder end of the embryo (Figs. 27, 30) ; the hindmost pair 

 passing into the, as yet, unsegmented mesoblast folds, which 

 end posteriorly in the two polar mesoblast cells (Figs. 30, 

 81, PC). 



At a stage when six pairs of somites are present, the 

 cavities of the anterior ones become constricted off from the 



P 



