94 THE EARLY DEVELOPMENT OF MAMMALS. 



and white, but various other cells outside of the vessels, of which two classes are 

 especially important the free wandering cells in the mesenchyma or connective 

 tissue and the giant cells. The latter, however, contribute to the blood, for they 

 form in the spleen, bone-marrow, and other organs, long processes, like pseudo- 

 podia, which break up into fragments. These fragments are the blood-plates. It 

 is probable that all blood-cells and wandering cells are exclusively descendents 

 of the primitive blood-cells, although some writers maintain that their number 

 is increased in both the embryo and the adult by transformation of cells of the 

 mesenchyma. 



When the circulation begins, the number of corpuscles is small, but it rapidly 

 increases by mitotic division of the cells. At the very start, like all cells produced 

 by segmentation of the ovum, the blood-cells are quite large, but they rapidly de- 

 crease in size until they reach the "first-stage," in which they appear as small 

 round cells (in the chick 8.3 to I2.5// in diameter) with a rounded granular reticu- 

 late nucleus and a minimal amount of protoplasm. In the next stage the amount 

 of protoplasm increases. We have next to consider separately the cytomorphoses of 

 the red and white corpuscles. 



Red Corpuscles. By examining the blood of chick embryos of successive ages 

 we can trace the- differentiation of the red cells. We find that the protoplasm 

 enlarges for several days, and that during the same time there is a progressive 

 diminution in the size of the nucleus, which, however, is completed before the area 

 of protoplasm reaches its ultimate size. The nucleus is at first granular, and its 

 nucleolus or nucleoli stand out clearly. As the nucleus shrinks, it becomes round 

 and is colored darkly, and almost uniformly, by the usual nuclear stains. This 

 change is called pyknosis. The blood-cells of mammals pass through the same 

 metamorphosis as those of birds. For example, in rabbit embryos of eight days 

 (Fig. 52, A) the cells have reached the stage with a granular nucleus and well- 

 developed cell-body. Corpuscles of this kind are characteristic of fishes and 

 amphibia, and they may, therefore, be designated as the ichthyoid cells. Two days 

 later the nucleus is already smaller, and by the thirteenth day has shrunk to its 

 final, dimensions. The cells in this condition are characteristic of the reptiles and 

 birds, and may be designated, therefore, as sauroid cells. The nucleated stage of 

 the cells is typical of embryonic life only in mammals. During the fetal period 

 the nuclei of the red cells gradually disappear and the cells are transformed into 

 the non-nucleated corpuscles, which occur only in mammals, so that this last may 

 be designated as the mammalian stage. The nuclei disappear by extrusion from 

 the cells. Usually they break into fragments, which are then expelled. Some- 

 times, though rarely, the nucleus goes out intact. The successive stages of the 

 blood-corpuscles in mammals illustrate the law of recapitulation (page 29). When 

 the nucleus disappears, the corpuscle becomes smaller. In the human embryo at 

 one month the red cells are the predominant blood-corpuscles. At two months 

 they are still the most numerous, although the non-nucleated corpuscles have begun 



