DEVELOPMENT OF THE NERVOUS SYSTEM 529 



traced into the region of the medulla oblongata of the adult. Ceph- 

 alad to these were six more or less irregular and transitory grooves. 

 His figures and description indicated considerable regularity in the 

 segmentation of the medulla oblongata and caudad to it, while 

 cephalad to it there is a greater irregularity. 



Description 



The neural plate up to closure. Several hundred eggs were 

 collected for this study. In the summer of 1911 the eggs then 

 collected were fixed in formalin^ Later these were examined and 

 camera sketches made of a number. Again in 1912 a considerable 

 number were collected, these being studied and sketched before 

 and after fixation. They were found to correspond very closely. 

 In several cases eggs were examined at periods in their growth and 

 the irregularities, described farther on, were found to persist. 



Figures 11 to 19 show typical specimens. I have portrayed the 

 actual conditions as accurately as possible and present these, 

 selected from a number of drawings, as being from representative 

 specimens. A sHght beading is indicated. Its irregularity in 

 size, number, and position makes its segmental significance very 

 questionable. 



In most of the specimens a depression extends across the caudal 

 portion of the procephalic plate. This I take to be the transce- 

 phalic groove of Griggs. Its position and prominence vary consid- 

 erably. In specimens younger than those figured, and less often 

 in the later stages before closure of the medullary plate, I have 

 been able to see transverse markings in the procephalic plate which 

 were very transient, varied in position in different individuals, 

 and which I was not able to trace through from one stage to 

 another in living specimens. Caudad to the transcephalic groove 

 the markings in the plate were more uniform and persisted longer 

 and were subject to but slight variation. In some specimens 

 they corresponded closely to the outpocketings in the medullary 

 folds but they failed to do this in other specimens. 



Another noticeable feature in embryos of this stage is the plica- 

 tions in the neural folds themselves. These are observed with 



