TUBULATION OF ORGAN-FORMING AREAS 465 



latter region, each rudiment of the dorsal aorta sends a small, vitelline blood 

 vessel laterally into that portion of the gut tube or yolk area containing the 

 yolk or other nutritional source. In the yolk area, each joins a plexus of 

 small capillaries extending over the surface of the yolk substance. These 

 capillaries in turn connect with other capillaries which join ultimately each of 

 the original subintestinal blood capillaries. Below the anterior or foregut por- 

 tion of the entodermal tube, the two subintestinal blood vessels fuse and thus 

 form the beginnings of the future heart (figs. 234-237; 332). The further 

 development of this system of primitive vessels is described in Chapter 17. 



7. Extra-embryonic Membranes 



Associated with the development of body form and tubulation of the major, 

 organ-forming areas, is the elaboration of the very important extra-embryonic 

 membranes. As the essential purpose at this time is to gain knowledge of the 

 changes concerned with tubulation of the major organ-forming areas and the 

 development of primitive body form, consideration of these membranes is de- 

 ferred until Chapter 22. The latter chapter is concerned with various activities 

 relating to the care and nutrition of developing embryos of various vertebrate 

 species. 



B. Tubulation of the Neural, Epidermal, Entodermal, and Mesodermal, 

 Organ-forming Areas in the Vertebrate Group 



1. Neuralization or the Tubulation of the Neural Plate Area 



a. Definition 



The separation of the neural plate material from the skin ectoderm, its mi- 

 gration inward, and its formation into a hollow tube, together with the segre- 

 gation of the accompanying neural crest cells, is called neuralization. 



b. Neuralizative Processes in the Vertebrata 



Neuralization is effected by two general procedures in the vertebrate 

 subphylum. 



1) Thickened Keel Method. In teleost, ganoid, and cyclostomatous fishes, 

 the neural plate material becomes aggregated in the form of a thickened, 

 elongated ridge or keel along the middorsal axis of the embryo (figs. 21 OF; 

 218C). This keel separates from, and sinks below, the overlying skin ectoderm 

 (fig. 218A). Eventually the keel of neural cells develops a lumen within its 

 central area and thus gradually becomes transformed into an elongated tube, 

 coincident with the tubulations of the other major organ-forming areas (fig. 

 218B). In the cyclostomatous fish, Petromyzon planeri, although neuraliza- 

 tion closely resembles the condition in teleost fishes, in certain respects the 

 behavior of the neuralizative changes represents an intermediate condition 



