cytes, usually quite early in their development. Nu- 

 merous macrophages are still present. 



Bone marroiv. — At this stage in the bone marrow 

 there is a variation from a very high percentage of 

 mature heterophils to relatively few, and red blood cells 

 are dominant. These two are always present simul- 

 taneously but the ratio varies at this age. 



1 DAY 18 HOURS POSTHATCHING 



Circulating blood. — At this age there is still a domi- 

 nance of heterophils and a general absence of the 

 agranulocytes in the circulating blood. 



Spleen. — At this period the dominant picture is 

 lymphoid with a variable number of heterophils, both 

 early and late stages. Early stages of eosinophil de- 

 velopment are present also. 



Bone marrow. — In one of the specimens there were 

 still a few primordial osteogenic cells but otherwise 

 there was a fairly constant picture of mature granulo- 

 cytes and red-cell developmental stages. 



4 DAYS 18 HOURS POSTHATCHENG 



Circlatiug blood. — In one chick there was a normal 

 blood picture and in the other there was still a pre- 

 dominance of granulocytes and a lag in lymphocyte 

 development. 



Bone marrow. — In both cases the bone marrow con- 

 tained a great many fully developed heterophils as 

 well as developmental stages of red cells. 



5 DAYS 18 HOURS AND 6 DAYS 

 POSTHATCHING 



Circulating blood. — The chick at 5 days 18 hours 

 showed an abnormal number of monocytes; lympho- 

 cytes were relatively few; the heterophils were not pre- 

 dominant; and there were some basophils. The 4 

 slides taken at 6 days show a fairly normal picture and 

 a fairly normal proportion of the different types of 

 white cells. 



Spleen. — In every case the dominant picture is lym- 

 phocytogenesis with only a few granulocytes develop- 

 ing. In some slides, stages in lymphocytogenesis are 

 well shown. 



Bone marrow. — All the bone-marrow slides show 

 about the same cell types as do those for the preceding 

 age. This is a good age for study of developmental 

 stages along the various lines. In one slide a throm- 

 boblast was clearly seen. 



8 DAYS POSTHATCHING 



Circulating blood. — In most of these smears there 

 was the usual ratio of white cells to red cells and the 



different types of white cells were present in their usual 

 proportions. In one chick there was a monocytosis 

 and many cells were smudged. This seems to be the 

 stage at which monocytes make their appearance ; some 

 of these monocytes show the typical reticular appear- 

 ance of the cytoplasm. 



Spleen. — Occasionally there are some late hetero- 

 phils in the spleen but it is now predominantly a lymph- 

 oid picture. 



The circulating blood, 7 days posthatching, is 

 illustrated in figure 231. The structural char- 

 acter of a lymphocyte, a monocyte, a heterophil, 

 and a basophil, as well as of some thrombocytes 

 and erythrocytes at this age, is shown. This pro- 

 portion of leukocytes to erythrocytes is greater in 

 the field selected for illustration than is usually 

 found. 



The spleen after its reorganization produces 

 blood smears of the type seen in figure 331, which 

 was from a chick 35 days posthatching. 



The statement has been made several times 

 that granulocytes held in hematopoietic organs of 

 the embryo are discharged into the circulation 

 soon after hatching. A contrary point of view 

 has been given by Nonidez (1920) from his 

 study on eosin-staining cells in the gonads of 

 bantams. They were abundant in the embryo 

 and began to disappear after hatching but he con- 

 cluded that the cells did not pass into the blood 

 stream, but underwent disintegration and were 

 taken up by special cell elements and endothelial 

 lining cells of blood vessels. In the protocol 

 numerous macrophages were noted in the spleen 

 at 1 to 3 hours, 8 hours, and 24 hours after hatch- 

 ing (which agrees with the observations made by 

 Nonidez on the tissues of the gonads), and 1 

 phagocytic cell was pictured in figure 317. From 

 the data thus far accumulated it would appear 

 that both gonad and spleen tissues are acting 

 similarly, but additional study is needed to de- 

 termine whedier the granulocytes of the embryo 

 organs after hatching are the definitive cells of 

 the circulating blood or whether all the masses 

 of embryo cells are destroyed and only those de- 

 veloped in the bone marrow after hatching reach 

 the circulating blood. DanschakofE (1916a) 

 observed in sectioned material what has been re- 

 ported here that, after hatching, earlier leu- 

 kopoietic functions are reduced and the spleen 

 becomes chiefly a lymphocyte-producing and an 

 erythrocyte-destroying organ. 



Plasma cells are rare in the normal chicken 



165 



