AMBLYSTOMA PUNCTATUM. T 



longitudinal axis with a slight groove or depression, the medullary 

 groove. The areas lying within the medullary folds on either 

 side of the medullary groove, are the medullary plates, and in 

 some instances, are composed of cells slightly larger and a trifle 

 lighter colored than those of the remainder of the embryo. 



The egg has meanwhile been changing shape, not only on the 

 dorsal side, that marked by the medullary folds, but also at the 

 anal end, in such a way that in a profile view of the latter region 

 there is seen a depression or a sinuosity in the outline, showing 

 that the originally spherical ovum is beginning to take on the 

 elongated form of tlie embryo. Plate 2, Figure 7. 



The medullary folds having become continuous, the process of 

 folding in and uniting with each other to form the closed, neural 

 tube advances with great rapidity; the entire process occupying 

 eight or nine hours. The first well-marked change in the folds, 

 alter they have become continuous at the cephalic end, takes place 

 at points in the lateral-walls about midway between the cephalic 

 and anal ends, where they grow inwards towards each other, 

 Plate 2, Figure 7 ; then the large, thick walls of the cephalic end 

 rapidly grow towards one another and unite over the middle line 

 of the medullary groove. Near the anterior ends, the cephalic 

 portion of the folds meet and unite first, the union gradually ex- 

 tending backwards along the median line. At the extreme ante- 

 rior end of the medullary folds however, a considerable space is left 

 which is the last to remain unclosed. In this way a fusiform space, 

 the sinus rhomboidalis comes to be left between the anal end and 

 a point about midway between the anal and cephalic ends, where 

 the folds first grew towards each other. Plate 2, Figure 8. This 

 fusiform space, though, is soon closed over by the advancing folds, 

 and is quickly followed by the closing over of the space left at the 

 cephalic end. At the extreme anal end, the folds remain separate 

 over a small area, the S|)ace formerly occupied by the vitelline 

 plug, and form a rounded edge about this small cavity or pit. It 

 becomes a definitely rounded cavity by the time that the first con- 

 striction, indicating the throat, is seen. While the neural tube 

 has been thus rapidly forming, the embryo has increased very 

 much in size, and its outline has become very much altered. It is 

 now much more elongated, and both the anal or caudal and 

 cephalic ends are becoming more definitely indicated as they grow 

 away or stretch out from the body of the embryo. The entire 



