63 



tube h, both enveloped by the wall of the body. Did the 

 row of cells x (or notochord) but reach a little farther forward 

 we should have an exact diagram of the section of a vertebrate 

 body. 



The neural cavity now becomes completely closed in, and 

 at the same time takes on more of a spherical shape. Within, 

 especially behind and underneath, a proliferation of cells 

 takes place, to a certain extent filling up the cavity. Pig- 

 ment is deposited in the form of a round patch which in time 

 becomes almost completely covered up by small cells. This 

 pigment patch is undoubtedly an organ of vision, and the 

 cells surrounding it constitute a ganglion. The upper and 

 anterior wall becomes very thin, and from the lower and 

 anterior wall there grows out a peculiar body not unlike a 

 wine-glass covered at its top by pigment, with a stem of 

 transparent, highly refractive, material. This body, believed 

 by Kowalevsky to be an auditory organ, projects freely into 

 the neural cavity. 



No trace of nerves leading from the neural cavity can be 

 detected. 



