192 HUBERT DANA GOODALE 



pore as a straight horizontal groove at the posterior end of tlie 

 neural plate (fig. 27) . As the neural folds begin to develop, the 

 blastopore continues to shorten and at the same time again be- 

 comes crescentic, but this time, its concavity is directed dorsally, 

 exactly opposite to its previous direction, plate 1, figs. 8 and 10. 

 The blastopore continues to shorten, sometimes becoming a 

 short thick, horizontal opening, or a short thick vertical slit 

 (plate 1, fig. 12), or even other shapes. At last however, it be- 

 comes a small rounded opening at the ends of the neural folds, 

 where it remains as the definitive anus. 



The numerous investigators into the fate of the amphibian 

 blastopore have failed to agree, but it appears thst in most Anura, 

 the anterior part of the blastopore, without closing becomes the 

 neurenteric canal, while the rest of the blastopore closes up. The 

 anus arises as a new invagination through the posterior part of 

 the fused space. In some Urodeles, e. g., Amblystoma, the 

 median parts of the slit-like blastopore fuse leaving an anterior 

 opening, the neuropore, and a posterior, the anus. In others, 

 e.g., Sperlepes, in which there appears to be no neurenteric canal, 

 the blastopore becomes the anus. 



Formation and closure of the neural folds 



About the time the blastopore becomes a straight line, the 

 neural groove appears. At first it is a wide, short and shallow 

 depression, just posterior to the present upper pole of the egg. It 

 is, then, near ihe original primary equator of the egg. It gradu- 

 ally deepens and then lengthens, until it forms a furrow, some- 

 times deep, sometimes shallow, extending over about 90°. Its 

 posterior end is usually near the equator. Sometimes it is con- 

 tinued by a dark stripe to the blastopore. 



The first appearance of the neural folds as two ridges, one on 

 each side near the equator, is shown in plate 1, fig. 7. Their 

 outer edges coincide with the boundaries of the neural plate which 

 can be distinguished as an elongated, slightly irregular ovoid. The 

 neural groove in this particular egg is narrow and extends from 

 a point near the anterior end of the neural plate back towards the 



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