532 p. E. SMITH 



more prominent at this stage than in any subsequent one. In this 

 specimen the fold in which the posterior commissure develops is 

 farther caudad than normal, thus lengthening the second dience- 

 phalic segment and shortening the first mid-brain segment. The 

 optic vesicles communicate more distinctly by the preoptic recess 

 than in the previous stage. 



^ Figures 31 and 32 are from a model of an embryo in a rather 

 late Stage IX. The velum transversum has become prominent, 

 as has also the infolding in which the posterior commissure will 

 develop. In the formation of these two constrictions, the one 

 separating the diencephalon into two parts has become less promi- 

 nent and is seen in the broad arch which forms the roof of the dien- 

 cephalon. The cells, however, show a typical neuromeric arrange- 

 ment. The mid-brain neuromeres have become more prominent 

 and a thinning in the roof of the medulla caudad to the cerebellar 

 neuromere is evident. Laterally the ridge separating the dien- 

 cephalon from the mesencephalon is much better shown than in 

 the previous stage. This appears to be due in its ventral portion 

 to the lateral expansion of the hypothalamus. As in the previous 

 models, the otic vesicle lies opposite the third and fourth medullary 

 neuromeres. The landmarks of the brain are now so distinct 

 that there can be no mistake about the identification of the various 

 parts. In the next model (figs. 33 and 34) all indications of the 

 neuromeres have disappeared. 



Discussion 



Upon the medullary plate and folds of the forms which have been 

 examined, it is evident that more or less regular ridges exist. 

 Locy, Hill, Smith, and Griggs have put forth evidence that they 

 have segmental value. Neal questions the observations of Locy; 

 Kupffer is surprised that the observations of Hill have been so 

 generally accepted. The form in which Griggs found transverse 

 markings upon the medullary plate Locy had previously regarded 

 as free from such markings, while observing a segmentation of the 

 neural folds. 



Caudad to and including that portion of the cephalic plate 

 entering into the medulla oblongata, investigators have found a 



