194 HUBERT DANA GOODALE 



wave-like elevation and movement of cells. It will be recalled 

 that the first appearance of the neural folds is near the equator. 

 A stained spot in this region, i.e., adjoining the epidermal ectoderm, 

 moves bodily to the dorsal mid-line. This is confirmed by sec- 

 tions, for the material of the neural plate ectoderm is composed 

 of fine yolk granules, while the cells of the epidermal ectoderm 

 are filled with much larger granules. When the neural folds have 

 closed, the cells containing the large granules from either side 

 have met in the dorsal mid-line. They now cover the entire outer 

 surface of the animal, except, perhaps, a small area immediately 

 in front of the head. Nearly all the material of the neural plate, 

 then, goes to form the nervous system of the adult. 



Discussion of the neural groove 



The shallow median groove described in previous sections 

 constitutes the so-called neural, median or dorsal groove. Some 

 authors apply the term neural or medullary groove to the entire 

 space separating the neural folds. Cross-sections of the neural 

 groove in early stages, show that in this region the ectoderm is 

 much thinner than at either side and is composed of a single layer 

 of cells which may be described as rounded, cubical. It lies 

 directly over the notochord which here forms the dorsal roof of 

 the archenteron. The true significance of this groove appears to 

 be entirely unknown. Hertwig ('92), Van Bambeke ('93), Braus 

 ('94, '02), Rothig ('01) consider it as the seam along which the 

 blastopore lips fused during the concrescence of the embryo. 

 Brauer ('97) points out that in Hypogeophis, in which there is 

 very little movement of the blastopore ventrally and in which the 

 embryo is well developed long before the blastopore closes, it 

 cannot represent such a seam. Schultze ('83, '88), Johnson ('84), 

 Morgan ('89), Erlanger ('90), have considered it, either in whole 

 or in part, as the primitive streak. Goette ('75) believes the 

 neural groove is due to the slower development of the notochord, 

 which fails to support the middle of the neural plate, thus per- 

 mitting this part to sag. Schwink ('84), Erlanger ('91), Robin- 

 son and Assheton ('91), Jordan ('93), Eycleshymer ('95) note 



