DEVELOPMENT OF THE NERVOUS SYSTEM 531 



them. In the later stages (fig. 18) the lateral irregularities do not 

 correspond to the medial ones. It would easily be possible to 

 select from the material a series of specimens which would show a 

 uniform development and fate of these foldings, but after the 

 examination of a large number of specimens, I have not been able 

 to satisfy myself that they have a definite fate. However, these 

 foldings can be traced through from the earliest appearance of the, 

 medullary folds to near the time of the fusion and formation of 

 the neural tube and they appear to be due to a rapidly lengthening 

 line of tissue upon a less rapidly lengthening base. At the beginning 

 of my study upon this problem I felt that each of the three charac- 

 teristics that I have described had a segmental significance and 

 studied them with the expectation of showing this as well as their 

 uniformity but have been forced to change my views because of 

 the evidence presented. 



The neural tube. Better to study segmentation in the neural 

 tube, several models have been made. Figures 27 and 28 are 

 from a model of an embryo in an early Stage VIII. The eye- 

 vesicles have not yet invaginated to form the optic cup and the 

 otic vesicles are still in broad comLmunication with the body ecto- 

 derm. 



In the medial view a very slight dorsal and lateral constriction 

 separates the fore-brain from the mid-brain. This infolding is not 

 well marked but can be definitely distinguished by the cell arrange- 

 ment. In the floor the folds marking where the anterior commis- 

 sure and optic chiasma will develop are prominent. It is evident 

 that the optic vesicles communicate with each other by both the 

 pre- and post-optic grooves. The mid-brain is separated into two 

 neuromeres by a very shght ridge. Posterior to the mid-brain a 

 neiu-omere is marked off by a pronounced constriction. This 

 neuromere later forms the cerebellum. The neuromeres of the 

 medulla are very prominent. 



Figures 29 and 30 are from a model of an embryo in a late' 

 Stage VIII. A shght constriction representing the velum trans- 

 versum has appeared. Between this and the infolding in which 

 the posterior commissure develops is a constriction which divides 

 the roof of the diencephalon into two segments. This infolding is 



