DEVELOPMENT OP THE TWO MIDDLE GERM -LAYERS. 



109 



the processes in Ampbioxus, Amphibia, Selachians, Birds, and Mam- 

 mals, since they differ somewhat from one another. 



The history of the development of Amphioxus lanceolatus is very in- 

 structive. The gastrula elongates, whereby the ccelenteron is turned 

 a little towards the future dorsal surface, and here terminates in the 

 blastopore, which marks the future hind end of the worm-shaped 

 body. Then the dorsal surface becomes somewhat flattened; the 

 cells in this region increase in height, become cylindrical, and form 

 the medullary or neural plate (fig. 69 mp). By a slight infolding of 

 the latter, there arises a medullary groove, which forces downward 

 the roof of the 



ccelenteron in ** dh us ' ush n ush mk m 



the form of a 

 ridge (ch). At 

 the place where 

 the thickened 

 medullary 

 plate joins the 

 small -celled 

 part of the 

 outer germ- 

 layer, or the 

 horn-layer (hb), 

 an interruption 



Fig. 68. Optical longitudinal [sagittal] section through an embryo of 

 Amphioxus with five primitive segments, after HATSCHEK. 



V, Anterior, H, posterior end ; it, inner, ink, middle germ-layer ; dh, 

 intestinal cavity ; n, neural tube ; en, neurenteric canal ; it* 1 , first 

 primitive segment ; nth, cavity of primitive segment. 



in the continu- 



ity now takes place, and the epidermis grows over the curved 

 neural plate from both sides, until its halves meet in the middle 

 line and fuse. Thus there arises along the back of the embryo 

 (fig. 70) a canal, the lower wall of which is formed by the curved 

 medullary plate (mp), and the upper wall by the overgrowing epi- 

 dermis (&). It is only at a later stage that the medullary plate in 

 Amphioxus, lying under the epidermis, is converted into a neural tube 

 (fig. 72 n) by the bending up of its edges and their fusion. As the 

 fundament of the nervous system becomes differentiated, it extends 

 so far toward the posterior end of the embryo, that the blastopore, 

 which is located there, still falls within its territory, and with the 

 closure of the neural tube is included within the end of the latter 

 Tn this manner it occurs that neural tube and intestinal tube, as 

 KOWALEVSKY first observed, are now, by means of the blastopore, 

 in continuity (fig. 68 'en) at the posterior end~bFthe body. The two 

 together constitute a canal composed of two arms, the form of which 



