52 



THE EARLY DEVELOPMENT OF MAMMALS. 



grows they begin to move apart, though remaining connected with one another by 

 protoplasmic processes. The cells separate least near the primitive streak, but their 

 separation becomes gradually more and more marked toward the periphery of the 

 layer, as shown in Fig. 19, which represents a part of the peripheral region of the 

 mesoderm of a blastodermic vesicle of a rabbit of seven days. 



In the details of its expansion the mesoderm varies greatly in different mammals. 

 In some forms it develops very early and rapidly expands over the entire blasto- 



m 



FIG. 18. CENTRAL PORTION OF A SHEEP'S BLASTO- FIG. 19. BLASTODERMIC VESICLE OF A RABBIT OF 



DERMIC VESICLE OF TWELVE TO THIRTEEN SEVEN DAYS. PORTION OF THE MESODERM 



- AYS - OF THE AREA OPACA. (After Kdlliker.) 



Sh, Embryonic shield, kn, Hensen's knot, mes, 

 Shadow caused by mesoderm developing around 

 the shield. X 34 diams. (After Bonnet.) 



dermic vesicle, which then becomes three-layered. This seems to be the method of 

 its growth in man and other primates. In other cases, as in the dog and cat, it 

 grows more slowly, but ultimately encloses the entire entoderm. In the rabbit, 

 on the contrary, it never expands more than about three fifths of the way over the 

 blastodermic vesicle, one part of which, therefore viz., that opposite the embryo 

 never has any mesoderm whatever. This, however, is to be regarded as a special 

 modification, since we must consider that primitively the mesoderm extended over 

 the entire vesicle. 



The Primitive Axis. 



The next stage of development is characterized by the appearance of an accu- 

 mulation of cells which extends forward from the primitive knot in the axial line. 



