96 THE EARLY DEVELOPMENT OF MAMMALS. 



to appear. At three months the non-nucleated corpuscles constitute by far the 

 majority of all corpuscles in the blood. 



Leucocytes. The primitive blood-cells, being colorless, have been termed leuco- 

 cytes by some authors, but they are obviously different from the leucocytes of the 

 adult blood. Some of them become so-called lymphocytes (young leucocytes), 

 which are distinguishable from the primitive cells by the internal structure of the 

 nucleus. Others grow in size and follow at least two cytomorphic paths. In one 

 series the protoplasm develops fine granules, and the nucleus becomes first elongate, 

 then reniform, and finally beaded. In this form they appear as polymorpho- 

 nuclear neutrophile leucocytes. In the second series the protoplasm acquires coarser 

 granules, which are really phagocyted morsels of red blood-cells, and the nucleus 

 becomes reniform. These cells are termed the eosinophile leucocytes. 



The term leucocyte properly embraces all the white corpuscles of the adult 

 blood, but has been erroneously restricted by some recent authors to the granular 

 forms. The young stages of the granular leucocytes are sometimes termed myelo- 

 cytes, because in the adult they occur chiefly in the bone-marrow, which is the 

 chief sanguifactive tissue in the adult. 



It is doubtful whether leucocytes ever develop in the circulating blood. They 

 appear abundantly after the lymph-glands are formed. The usual explanation is 

 that some of the wandering primitive blood-cells enter the glands, there multiply, 

 and in part become leucocytes. 



The Origin of the Heart. 



The manner in which the head of the embryo becomes free is described on page 

 49 (compare also Figs. 16 and 132). The origin of the ccelom is described on page 

 8 1. When the head becomes free, the ccelom is found soon to extend across the 

 median line. This takes place at the cervical end of the head just where the 

 tissues of the embryo bend over to join the yolk (Fig. 132, p. 183). This median 

 ccelom is the beginning of the pericardial cavity. In connection with it the 

 development of the heart occurs. The formation of this organ is probably initiated 

 by an ingrowth of the cells of the angioblast, which give rise to the endothelium 

 of the heart (Fig. 138, Endo). The mesothelium of the dorsal side of the primitive 

 pericardial coelom produces the muscular walls of the heart (Fig. 129, m.ht). The 

 early development and primitive relations of this organ can be understood by the 

 account given in Chapter V of the structure of a chick embryo with eight 

 segments. 



The Germinal Area. 



The germinal area is that portion of the amniote ovum (mammalian blastodermic 

 vesicle) in the center of which the embryo is differentiated. It comprises, therefore, 

 both the embryo proper and the region immediately surrounding it. In mammals 

 it corresponds in extent with the embryonic shield (p. 47). In its center we find 



