.4jV^-irOMy AND DEVELOPMEXT. 



199 



medullary plate occur simultaneously, so that when the 

 latter has the form of a half-canal it is not closed over 

 by a layer of ectoderm, but is open to the exterior 

 (Fig. 98). 



At a somewhat later stage the two medullary folds meet 

 together and fuse in the middle line (Fig. 98 E), and this, 

 combined with a slight forward growth of the posterior 

 lip of the blastopore, leads to the inclusion of the latter 

 in the medullary tube, 

 so that we arrive at the 

 condition already de- 

 scribed for Amphioxus, 

 in which the nerve -tube 

 opens in front to the 

 exterior by the nejiropore 

 and behind into the ar- 

 chenteron by the blasto- 

 pore, which has now 

 become converted into 

 the neiirenteric canal. 



Meanwhile the cells 

 forming the dorsal wall 

 of the archenteron in its posterior two-thirds begin to 

 gather themselves together to form the notochord (Figs. 

 98 and 99). The cells forming the notochord are at first 

 arranged end to end (Fig. 99), and subsequently interlace 

 in the manner described above for Amphioxus. 



Fig. 99. — .-/. Embryo of Phallusia mam- 

 viillata seen in optical .section from above, to 

 show notochord. 



B. Section through tail of older embryo 

 of Phallusia mammillata. (After KOWALEV- 

 SKV.) 



ch. Notochord. ec. Ectodenn. en. Endo- 

 derm. mes. Mesoderm, nt. Medullary tube. 



Origin of Mesoderm. 



At about the same time in which the formation of the 

 medullary tube and notochord is going on, the mesoderm 

 begins to put in its appearance, and this is the first event 

 in the development in which there is an important dif- 



