Spleen. — Apparently at the time of hatching, and 

 very rapidly thereafter, the spleen discharges its hetero- 

 phils; thus, in this period of 1 to 3 hours, one can find 

 a spleen filled with numerous mature heterophils; in 

 other cases, at the same age, there will be very few 

 heterophils, and in their place will be early eosinophil 

 metagranuloblasts, and even the early lymphoid se- 

 ries is making its appearance. At this period of transi- 

 tion it is quite common to see numerous macrophages 

 in the spleen. At this age and even up to 24 hours of 

 age there are many opportunities to study the develop- 

 ment of the eosinophil and the lymphocyte. 



Bone marrow. — Of the 4 chicks used at this age, 2 

 showed numerous late heterophils, and 2 did not. There 

 appears to be a transitional change in the bone mar- 

 row. It probably begins somewhat earlier than 1 to 

 3 hours after hatching, because the bone marrow is 

 already taking over the hematopoietic function in re- 

 gard to granulocytes that the spleen is giving up. In 

 addition, in the bone marrow one finds numerous ex- 

 amples of the red cells from early to late stages of de- 

 velopment. Mononucleated osteoclasts were always 

 present in these early stages, and were particularly 

 numerous in one case where very few heterophils were 

 present in the bone marrow. Osteoblasts also were 

 visible, but in those smears where heterophils are nu- 

 merous, osteoblasts are relatively rare. The third cell- 

 ular element coming up at this time is the thrombocyte 

 series. Just as early erythroblasts may be found, so 

 also may early thromboblasts be identified. These are 

 relatively large cells and have a characteristic punctate, 

 strongly granular nucleus that stains more intensely 

 than the erythroblasts. Most of these cells, however, 

 contain a well-developed nucleolus. This structure 

 aids in the identification of the cell, but in the throm- 

 boblast it is often masked and covered over by strongly 

 stained chromatin spheres. The thrombocyte as it gets 

 older has, typically, a very dark blue cytoplasm and 

 frayed edges. In these early bone marrow smears 

 the transitional stages down to the small thrombocyte 

 can be followed. The fully formed thrombocyte, how- 

 ever, does not have the form present in the circulating 

 blood because of degeneration of the cytoplasm that 

 takes place during fixation. Cells of the thrombocyte 

 series are never numerous in the bone marrow but 

 some are always present. 



8 TO 12 HOURS POSTHATCHING 



Circulating blood. — At this stage there may be some 

 holdover of what was found at 1 to 3 hours, because the 

 dominant type of cell among the white blood cells is the 

 heterophil. Sometimes the white blood cells are few 

 in relation to the red cells and sometimes they are at 

 about the normal ratio. At this age the number of 

 heterophils is generally out of proportion to the other 

 white blood cells and the lymphocytes and monocytes 

 are definitely fewer than they will be later. It is char- 

 acteristic of the circulating blood of the recently 

 hatched chick that the red blood cells show a wide range 

 of immature stages of development, but predominantly 



the stages most commonly seen begin at about the mid- 

 polychromatic erif-throcytt and continue on to maturity 

 (fig. 230) . One gathers the impression by comparing 

 the development of red blood cells as seen in the bone 

 marrow with that seen in the circulating blood that 

 there is a tendency for the bone marrow to throw out 

 the cells at about the mid-polychromatic erythrocyte 

 stage or a little earlier. Later stages are present in the 

 bone marrow but cells of older ages do not dominate 

 the picture. 



Spleen. — In the 4 chicks used at this age (8 to 12 

 hours) there was a wide range in the number of hetero- 

 phils but in general they had almost completely dis- 

 appeared and in their place were numerous eosinophils 

 in about the mid-stage of development. Many macro- 

 phages were present and there was an increase in num- 

 ber of small lymphocytes. In another chick at this 

 same age there was a moderate number of mature het- 

 erophils with developing eosinophils and developing 

 basophils; in the third chick there were numerous early 

 heterophils, practically no eosinophils, and some early 

 basophils; and in the fourth there was a mixture of 

 mid-heterophils and early eosinophils. In spite of this 

 variability, there still exists an extensive discharge of 

 the heterophils into the circulating blood. This is fol- 

 lowed by a wave of development of eosinophils and per- 

 haps in some cases an additional group of heterophils 

 and basophils. At this time, also, there is some indi- 

 cation that the lymphocytes are taking the dominant 

 place that they will hold later in the spleen. 



Bone marrow. — In the bone marrow of all these 

 chicks (8 to 12 hours) there were a great many mature 

 heterophils. This seems to be characteristic of the 

 bone marrow at this stage and later. Almost equally 

 conspicuous are the developing red blood cells and the 

 scattered thrombocytes. Some small thrombocytes are 

 present also, and one can find the developmental stages 

 in the production of granulocytes, but these are mingled 

 with the adult stages and are not so conspicuous as they 

 are in the spleen. 



24 HOURS POSTHATCHING 



Circulating blood. — At 24 hours the white blood cells 

 in the circulating blood are predominantly heterophils. 

 Other cell t}'pes are present, of course, but they have 

 not reached their usual ratio and, in many cases, lym- 

 phocytes or monocytes, or both, may be lacking or at 

 least be so scarce that a great deal of searching is re- 

 quired to find them. At this stage there are abundant 

 examples of late-developing red cells. 



Spleen. — Only in 1 of the 4 chicks at this age was 

 there even a moderate number of mature heterophils 

 in this organ. At this stage the lymphocyte develop- 

 ment is coming into view. This is an excellent stage 

 at which to study lymphopoiesis: some of the cells can 

 be traced back to lymphoblasts and to the reticular 

 cells from which they appear to have come. Among 

 these early lymphoid cells are some eosinophil myelo- 



164 



