112 



DEVELOPMENT OF AMPHIOXUS. 



while the opposite side is flattened ; this is an indication 

 that dorso-ventral differentiation has taken place, since 

 the flattened side marks the dorsum or back of the embryo, 

 while the convex side is ventral. It may be seen further 

 that the blastopore is inclined towards the dorsal side of 

 the embryo. The dorsal inclination of the blastopore is 

 eminently characteristic of the vertebrate gastrula from 

 the Ascidians up to the highest 

 craniate forms. In the Inverte- 

 brates (Annelids, Molluscs, etc.) 

 the blastopore acquires a ventral 

 inclination.* 



At the stage represented in Fig. 

 60 the embryo commences to ro- 

 tate within the vitelline membrane, 

 each ectodermic cell being now 

 provided with a vibratile cilium. Fig. 61. Elongated gas - 



,, . . trula. Optical longitudinal sec- 



The embryo next begins to elon- tion- The cilia are omitted 



gate, and the blastopore becomes from the ectoderm - (A fter 



HATSCHEK.) 



still narrower (Fig. 61). 



A comparison of the accompanying figures will show 

 that the narrowing of the blastopore is effected by the 

 downward and backward growth of its dorsal border, 

 while the ventral lip remains stationary. The dorsal ecto- 

 derm, which is converted into the medullary plate, now 

 shows indications of a shallow longitudinal groove. This 

 is the beginning of the medullary groove which leads on 

 to the formation of the central nervous system. 



* For a discussion of the phylogenetic relation of the blastopore or proto- 

 stoma (Hatschek) to the mouth and anus, the following works should be 

 consulted: ADAM SBDGWICK, On the Origin of Metameric Segmentation, etc., 

 Quarterly Jour. Micro. Sc., XXIV., 1884, and by the same author, Notes on 

 Elasmobranch Development, Ib. Vol. XXXIII., 1891-92. 



Finally, BERTHOLD HATSCHEK, Lehrbuch der Zoologie, Jena, 1888-91. 



