Early Development of Desmognathiis Fusca. 545 



small nearly straight blastopore. Very soon this neural plate with 

 its groove becomes triangular (Figs. 30 and 31). The depression 

 of the neural groove becomes considerably deeper, definite folds are 

 produced on each side with an open space in front; these are thick 

 and higher in front or at the end away from the blastopore. Running 

 down between them is the narrow median groove which was less 

 prominent in early stages. In Fig. 31, the blastopore appears as 

 a little triangular opening at the caudal extremity of the depression. 

 The neural folds have not reached as far as this. 



In later stages, such as in Fig. 32, the neural folds become con- 

 tinuous at the head end, the outline of the folds is elliptical, they 

 change soon to become more rounded as they become higher and the 

 outline of the cephalic end becomes more circular or truncate-ellip- 

 tical, Fig. 33. 



When the medullary folds begin to be prominent, the mesoderm is 

 easily recognized as composed of two layers. It could not be traced 

 very far toward the ventral side. In the cephalic region of the em- 

 bryo figured in section, Fig. 61, the mesoderm is separated in two 

 parts by the middle line of the notochord. Farther back in the same 

 embryo there is unmistakable evidence that the notochord is formed 

 directly from the dorsal portion of the archenteron (Fig. 63). 



Later stages of medullary plate formation, such as Figs. 35, 36, 37, 

 and 38, show the way in which the opposite neural folds meet, in the 

 caudal region first and later in the head region. The embryo in these 

 early stages seems to be quite well elevated from the yolk, the head 

 end is rather broad from the first. Soon myotomes may be seen a 

 little distance back from the head. The eyes bud out from the brain, 

 gill arches are formed and the embryo comes to encircle more and 

 more of the yolk as it grows in bulk and becomes differentiated (Figs. 

 40, 41, and 42). In quite a little later stage, such as Figs. 44, 45, 

 46, the head is well formed, the brain cavities may be seen through 

 the transparent mass of the head, the myotomes are numerous and 

 the head and tail ends of the embryo may be seen on the same side 

 of the egg. 



In a later stage such as sho^vn in Figs. 47 and 48, the embryo 

 becomes partly coiled about the egg, the eye, nasal pit, heart and 



