84 THE EARLY DEVELOPMENT OF MAMMALS. 



stage the primitive cells are found to have acquired connection with one another, 

 the protoplasm of one cell uniting by a process, or prolongation, with the proto- 

 plasm of another cell, and so on until the whole tissue becomes a network. When 

 the primitive streak has been formed in the mammalian blastodermic vesicle, 

 we find the mesoderm in this condition. The third stage is brought about by 

 the development of the coelom as above described, and it seems probable that all 

 the cells of the mesoderm are transformed into mesothelium. But this prob- 

 ability is not at present wholly beyond question. It is certain that nearly, if not 

 quite, all the mesodermic cells become mesothelium. To produce the fourth 

 stage, single cells leave the mesothelium or migrate out of it on the side away 

 from the coelom. These cells are found to be connected both with one another 

 and with the mesothelial cells by protoplasmatic processes, but they do not lie 

 close together, as in the epithelium, so that there is a considerable, though vari- 

 able, amount of intercellular space. By the migration of the cells and their 

 multiplication, the mesenchyma is produced. It fills up all the room between 

 the mesothelium and the two primary germ-layers so far as it is not occupied by 

 the developing blood-vessels. 



Apparently the entire mesothelium may participate in the production of the 

 mesenchymal cells. Its different regions, however, do not so participate all to 

 an equal degree, or at the same time. The throwing off of mesenchymal cells 

 may be observed in certain parts of the embryo in somewhat advanced stages of 

 development, and it seems not impossible that the process may be found to 

 occur even in adult life. 



The mesoderm, by the formation of mesenchyma, becomes very early unlike 

 the other germ-layers. Both ectoderm and entoderm are epithelial membranes. 

 The mesoderm is partly epithelial, partly mesenchymal, and from the mesen- 

 chyma arise special kinds of tissue which are characteristic of the middle germ- 

 layer, and never are produced from either the outer or inner germ-layers. 



The Germ=cells. 



Concerning the primitive origin of the germ-cells in vertebrates our knowl- 

 edge is scanty. The most accurate information we have refers to their develop- 

 ment in the dog-fish. In this species the germ-cells are delaminated from the 

 entoderm together with other cells of the mesoderm, and cannot, with our present 

 knowledge, be distinguished from other mesodermic cells. They soon, however, 

 became recognizable, because while the majority of the mesodermic cells are 

 passing into the second stage (compare the section on Mesenchyma, page 83) 

 these germ-cells change but little, if at all, so that they can be recognized as 

 something distinct from the neighboring cells. For a short time they are found 



