CHAPTER VIII 



THE CHANGES BETWEEN TWENTY-FOUR AND THIRTY- 

 THREE HOURS OF INCUBATION 



The closure of the neural tube; the differentiation 

 of the brain region; the anterior neuropore; the 

 sinus rhomboidalis; the fate of the primitive streak; 

 the lengthening of the fore-gut; the appearance 

 of the heart and omphalomesenteric veins; organ- 

 ization in the area vasculosa. 



In dealing with developmental processes the selection of 

 stages for detailed consideration is more or less arbitrary and 

 largely determined by the phenomena one seeks to emphasize. 

 There is no stage of development which does not show some- 

 thing of interest. It is impossible in brief compass to take up 

 at length more than a few stages. Nevertheless it is important 

 not to lose the continuity of the processes involved. By calling 

 attention to some of the more important intervening changes, 

 this brief chapter aims to bridge the gap between the 24-hour 

 stage and the 3'3-hour stages of the chick both of which are 

 taken up in some detail. 



The Closure of the Neural Tube. — In comparison with 24- 

 hour chicks, entire embryos of 27 to 28 hours of incubation 

 (Fig. 18) show marked advances in the development of the 

 cephaHc region. The head has elongated rapidly and now pro- 

 jects free from the blastoderm for a considerable distance, with 

 a corresponding increase in the depth of the subcephaUc pocket 

 and in the length of the fore-gut. 



In 24-hour chicks the anterior part of the neural plate is 

 already folded to form the neural groove. Although the neural 

 folds are at that stage beginning to converge mid-dorsally the 

 groove nevertheless remains open throughout its length (Fig. 

 17, Af B, C). By 27 hours the neural folds in the cephaUc 

 region meet in the mid-dorsal hne and their edges become fused. 



The fusion which occurs is really a double one. Careful 

 following of Figures 26, A to E, will aid greatly in understanding 



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