94 THE EARLY DEVELOPMENT OF MAMMALS. 



The cells are at first granular and slightly colored. As the protoplasm increases 

 about each nucleus the cell-body becomes more distinct, more colored with 

 hemoglobin, and more homogeneous. 



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 proto- 

 plasm reaches its ultimate size. The nucleus is at first granular, and its nucleo- 

 lus or nucleoli stand out clearly. As the nucleus shrinks, it becomes round and is 

 colored darkly, and almost uniformly, by the usual nuclear stains. The blood- 

 cells of mammals pass through the same metamorphosis as those of birds. For 

 example, in rabbit embryos of eight days (Fig. 4.2, 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 this may, therefore, be 

 designated as the ichthyopsidan stage. Two days later the nucleus is already 

 smaller, and by the thirteenth day has shrunk to its final dimensions. This 

 condition of the corpuscles is characteristic of the reptiles and birds, and may be 

 designated, therefore, as the sauropsidan stage. 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 successive stages of the blood-corpus- 

 cles in mammals illustrate the law of recapitulation (page 41). There has been 

 much discussion as to the manner in which the nucleus disappears in order to 

 convert the nucleated cell into the non-nucleated mammalian blood-corpuscles, 

 but authorities are not yet agreed. When the nucleus disappears, the corpuscle 

 becomes smaller. In the human embryo at one month, the red cells are the only 

 blood-corpuscles. At two months they are still the most numerous, although 

 •the non-nucleated corpuscles have begun to appear. At three months the non- 

 nucleated corpuscles constitute by far the majority of all corpuscles in the blood. 



Leucocytes. — The origin of the first leucocytes in the embryo is still uncertain. 

 Blood is found to contain for some time only the red cells, the leucocytes not 

 appearing in the chick until about the eighth day, in the rabbit about the ninth, 

 and in the elasmobranchs not until the embryo is well advanced in its develop- 

 ment. It is generally believed that the leucocytes do not arise in the blood- 

 vessels, and that they have no genetic relationship to the red blood-corpuscles. 

 It is probable that leucocytes are of several kinds, and of several distinct origins. 



