THE EMBRYONIC SHIELD. 



47 



The interior of the vesicle is filled with fluid. As the vesicle grows the fluid in- 

 creases in amount, and is presumably derived by the ovum from the walls of the 

 uterus. It is under pressure within the vesicle, as is shown by the manner in which 

 it spurts out if the vesicle is broken. Nothing exact as to the composition of this 

 fluid is known, though we may suppose it to resemble more or less the serous fluid 

 of the adult body. The size and form of the vesicle offer characteristic variations 

 in mammals. It starts as a more or less nearly spherical body. In the rabbit it 

 assumes an oval shape, and by the seventh day measures about 4.0 mm., and soon 

 thereafter becomes attached to the wall of the uterus. In the hedgehog, the guinea- 

 pig, and the mouse the ovum, while very small and more or less rounded in form, 

 becomes imbedded in uterine tissue and develops into a special shape in adapta- 

 tion to its new situation. In the ungulates the vesicle grows enormously, becoming 

 a very long and slender sac. Thus, for example, in the sheep it may measure 

 on the fourteenth day not less than 50 cm. in length. 



Another respect in which the blastodermic vesicles differ greatly from one an- 

 other in various mammals is in regard to the early development of the subzonal 

 layer, or, as we may call it, the ectoderm. In many cases the entire layer under- 

 goes a precocious development, its cells multiply very rapidly, so that the layer 

 becomes several cells thick. This thickened layer is known as the trophoderm. In 

 other placental mammals this thickening is confined to a limited area of the ecto- 

 derm. For further description see Trophoderm, page 114. 



The Embryonic Shield. 



Sooner or later in the early history of every blastodermic vesicle, and always 



as the first indication of the development of the embryo proper, there appears a 



thickening of a small oval area of the outer layer in the region of the inner mass. 



FIG. 13. TRANSVERSE SECTION THROUGH THE EMBRYONIC SHIELD OF THE BLASTODERMIC VESICLE OF A DOG 



OF ELEVEN OR FIFTEEN DAYS (PREQISE AGE UNKNOWN). , 



O.L, Outer layer. Ent, Entoderm. X 2 oo diams. (After Bonnet.) 



This thickening is known as the embryonic shield. In the fresh specimen it marks 

 itself by the greater opacity which it causes in the walls of the ovum where it lies. 

 In those cases where the thickening of the ectoderm to form the trophoderm ex- 

 tends over the entire blastodermic vesicle, it is very difficult to follow the early 

 history of the embryonic shield. In other cases, however, where the trophoderm 

 occupies a special restricted area, the history of the embryonic shield may be more 

 readily followed. The animals in which it has hitherto been chiefly studied are 



