60 AMPIIIOXUS. 



through the posterior part of an embryo (Fig. 23) will show a more 

 advanced stage in the formation of the nervous system than one 

 taken through the same embryo nearer its anterior end (Fig. 22). 



At the time of hatching (Fig. 25), the closure of the neural 

 tube is completed along about a third of the length of the 

 embryo ; the anterior opening of the tube, just in front of the 

 reference line NF in the figure, is spoken of as the neuropore. 



The mesoblastic somites. During the formation of the neural 

 canal important changes take place in the hypoblast. The- 

 flattening of the dorsal surface of the embryo at the completion 

 of the gastrula stage affects the hypoblast as well as the epiblast 

 (cf. Figs. 19 and 21). As the neural plate becomes marked off 

 and depressed, a pair of longitudinal folds of the wall of the 

 archenteron are formed, one along each side, in the angle 

 between its dorsal and lateral walls (Figs. 22 and 23, CE). These 

 folds are at first very inconspicuous, but rapidly become more 

 prominent, and especially so about the time of closure of the 

 neural canal (Fig. 24, CE). 



By the formation of these folds the archenteron becomes 

 divided into three portions : a central division (Fig. 24, G), which 

 is the alimentary canal itself, and a pair of lateral slit-like 

 diverticula (Fig. 24, CE), which may be termed enteroccelic 

 cavities, and which later on give rise to the body cavity or 

 coelom of the adult. 



The cells composing the walls of these folds are clearly of hypo- 

 blastic origin. In the later stages (cf. Figs. 27, 28, and 29), 

 they separate completely from the wall of the alimentary canal, 

 and are then spoken of as forming a third germinal layer, or 

 mesoblast, situated between the two primary layers, epiblast and 

 hypoblast. 



The mesoblastic folds extend the whole length of the embryo ; 

 they are most prominent near its anterior end, and gradually 

 diminish posteriorly, becoming continuous at their hinder ends 

 with the two large polar mesoblast cells (Figs. 25 and 26, PE), 

 which have already been described as present in the posterior 

 lip of the gastrula from an early stage in its formation (Fig. 18). 



Soon after their appearance, the mesoblastic folds become 

 divided by transverse constrictions into segments or compart- 

 ments, the mesoblastic somites, arranged in pairs along the sides 

 of the embryo. The anterior pair of somites, which are the first 



