CHAPTER VIII. 

 DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



General Form. 



The vertebrate body is fundamentally cylindrical. The trunk is con- 

 tinued forward into the neck which in turn supports the head. The extremi- 

 ties are appendages of the trunk. This form arises during the development of 

 the organism as a whole from the spherical egg cell. In Amphioxus the 

 spherical form is retained until the gastrula begins to elongate; in the frog 

 the same is true. In both these animals the simple elongation of the gastrula 

 is the first step in the change to the cylindrical shape. In the bird the egg is 

 spherical, but the cytoplasmic portion of the egg is a disk and out of this 

 disk the early cylindrical body is established by a process of folding. The 

 mammalian ovum also is spherical, but the part of the structure resulting 

 from the early processes of development which gives rise to the body is a disk; 

 and out of this disk the cylindrical body arises by folding in much the same 

 manner as in the bird. 



Since cleavage and the formation of the blastodermic vesicle in man has 

 not been observed, it is necessary to take some other mammalian form for 

 the early stages. In most mammals cleavage results in a solid mass of cells 

 called the morula (Fig. 55, a). In certain forms, like the bat, the superficial 

 cells of the mass become differentiated from those in the interior, the result 

 being an enveloping layer and a central mass (Fig. 55, b). In the opossum 

 during cleavage the blastomeres arrange themselves around a central cavity 

 so that no definite morula is formed (Fig. 57) . In the case of the solid sphere, 

 vacuoles appear within the central cells and then coalesce to establish a large 

 cavity which occupies the greater part of the interior of the sphere. There 

 remain then the enveloping layer and a few of the central cells which are 

 attached to the enveloping layer over a small area and which comprise the 

 inner cell mass (Fig. 55, c, d) . The cavity of the sphere in the mammal is prob- 

 ably not homologous with the blastocoel in the lower forms. The vacuoliza- 

 tion of the central cells has been interpreted as an attempt at yolk formation. 

 Whether the interpretation is correct or not, the cells surrounding the cavity 

 behave in many respects as if yolk were present; and the cavity subsequently 

 becomes the cavity of the yolk sac of the embryo. 



Following the formation of the yolk cavity, the contiguous cells of the 

 inner cell mass proliferate and migrate to form a complete lining for the 



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