CHAPTER IX 



THE STRUCTURE OF CHICKS BETWEEN THIRTY-THREE 

 AND THIRTY-NINE HOURS OF INCUBATION 



THE DIVISIONS OF THE BRAIN AND THEIR NEUROMERIC STRUC- 

 TURE; THE AUDITORY PITS; THE FORMATION OF EXTRA-EM- 

 BRYONIC BLOOD VESSELS; THE FORMATION OF THE HEART; 



THE FORMATION OF INTRA-EMBRYONIC BLOOD VESSELS. 



Chicks which have been incubated from 33 to 39 hours are 

 in a favorable stage to show some of the fundamental steps in 

 the foimation of the central nervous system, and of the circu- 

 latory system. In this chapter, therefore, attention has been 

 Concentrated on these two systems. 



During this period of incubation there are also changes in 

 the fore-gut region and in the somites, and differentiation in 

 the intermediate mesoderm which presages the formation of 

 the urinary organs. Consideration of these structures has, 

 however, been deferred until their development has progressed 

 somewhat farther. 



The Divisions of the Brain and Their Neuromeric Structure. 

 The metameric arrangement of structures which is so striking 

 a feature in the body organization of all vertebrates, is masked 

 in the head region of the adult by superimposed specializations. 

 In the brain of young vertebrate embryos, however, the meta- 

 merism is still indicated. Dissections of the neural plate of 

 chicks at the end of the first day of incubation show a series of 

 eleven enlargements marked off from each other by contric- 

 tions (Fig. 20, A). Concerning the precise homologies of indi- 

 vidual enlargements with specific neuromeres in other forms 

 there is not complete agreement. The controversies center 

 about the question of neuromeric fusions in the anterior part 

 of the brain. For the beginning student the fact that meta- 

 merism is present is to be emphasized rather than the contro- 

 versies concerning the homologies of neuromeres. With the 

 reservation that some of the anterior enlargements may repre- 



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