DEVELOPMENT OF THE NERVOUS SYSTEM 513 



Stages 



It has not been possible to raise this form in the laboratory, so 

 that the ages of the larvae have not been ascertained and conse- 

 quently have not been available for the classification into stages. 

 The body length, also, has been of but little direct value. More use 

 could not be made of it because the larvae flex in two planes as well 

 as undergoing an axial rotation. Thus it is only after hatching 

 that lengths can be given. In the description under the various 

 headings the different characters used to determine the stage will 

 be given. This can be supplemented by consulting figures 15 to 

 26. The specimens portrayed in these drawings were selected 

 from a large amount of material as being the most normal. 



CLOSURE OF THE NEURAL TUBE 



The dense yolk mass appears to affect strikingly the closure of 

 the neural tube. Its effect upon some of the earlier processes 

 before the formation of the nervous system is noted by Hilton 

 ('09) . In his conclusions is the following : ' ' The blastopore forma- 

 tion is much like other Amphibia from the surface but seems to be 

 entirely different from other forms described in the way the cells 

 go to form the archenteron by growing down into a nearly solid 

 mass of yolk." In contrast to most Amphibia in which the cavi- 

 ties are well marked, this species, due to the influence of this dense 

 yolk mass, has the various cavities, including the neurocoele, 

 greatly reduced. In the formation of the neural tube the medul- 

 lary folds first meet in the hind-brain region. From this point 

 closure takes place both caudad and cephalad and in a manner 

 typical of some of the lower forms, eg., Bdellostoma. That is, 

 from the hind-brain caudad only a morphological not an actual 

 neurocoele is present. The actual cavity forms intrasegmentally, 

 first appearing in the center of each of the segments, and these 

 gradually enlarge and coalesce, this process proceeding from the 

 head caudally. 



At the time of the closure of the neural folds and as late as 

 Stage VII it is not possible to distinguish the three germ layers in 

 the tail region, all being fused in a caudal mass which appears to be 



