64 



THE EARLY DEVELOPMENT OF MAMMALS. 



thin layer of cells very closely connected with the mesoderm. The mesoderm 

 occupies at first only a small area in the immediate neighborhood of the primitive 

 streak. It grows rapidly, so that its edge extends further and further over the 

 blastodermic vesicle. The mesoderm is to be regarded as the product of the 

 entoderm. Its exact origin in mammals has not yet been adequately traced. 

 We know, however, that in birds, reptiles, and elasmobranchs the cells of the 

 inner layer multiply rapidly, so that the inner layer becomes more than one 



FIG. 20. CENTRAL PORTION OF A SHEEP'S BLASTO- 

 DERMIC VESICLE OF TWELVE TO THIRTEEN 

 DAYS. 



S/i, Embryonic shield. Kn, Hensen's knot, vies, 

 Shadow caused by mesoderm developing around 

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



FIG. 21. BLASTODERMIC VESICLE OF A RABBIT 

 OF SEVEN DAYS. PORTION OF THE MESO- 

 DERM OF THE AREA OPACA. (After Kdlliker.} 



cell thick. The upper cells soon split off from the lower and thus form them- 

 selves into the middle germ-layer. The mesoderm therefore is said to be 

 formed by delamination. It seems probable that in mammals the process is 

 the same. 



It may be mentioned that, according to Bonnet, the development of the 

 mesoderm is not quite as above described. It can be first distinguished at the 

 stage when the primitive knot has appeared, and before the primitive streak is 

 developed. In the fresh specimen it is seen as a slight turbidity of the vesicular 



