56 CHARLES R. STOCKARD 



The only cells within the embryo which resemble lymphocytes 

 or leucocytes in their general structure and staining capacities 

 have been found in the anterior portions of the body and in the 

 head region of the young embryos. In very young embryos of 

 seventy-two hours, numerous isolated cells and occasionally 

 small groups of cells are found within the mesenchyme which 

 present a peculiar appearance. The nuclei are more or less 

 dense, the cytoplasm very small in amount in many and in 

 others very extensive, and staining with a color quite different 

 from that of other cells in the embryo. 



Figure 45 shows a section through the head just behind the 

 optic stalk of an embryo of seventy-two hours. In this section 

 there is seen a nest of the above-mentioned cells, several are 

 polynuclear and present various peculiar appearances. The 

 mesenchyme within this region is in active mitosis. 



Figure 46 is also taken from the anterior end of an embryo and 

 shows two large mesenchymal nuclei with numerous small leu- 

 cocyte-like cells within the mesenchyme. Numerous pigment 

 granules are also present in these mesenchyme cells. Some of 

 the cells present nuclei of the polymorphonuclear type. 



Figure 47 shows an enlarged binuclear cell and indicates the 

 fine granular nature of the cytoplasm. Such cells resemble 

 very closely the embryonic white blood cells. 



Figure 48 represents a section through the anterior end of 

 an embryo, and shows an endothelial artery which is entirely 

 empty of blood cells. Within the mesenchyme, near the ves- 

 sel, are two of the leucocyte-like cells. Other cells in these em- 

 bryos resemble very closely ordinary lymphocytes and these 



Fig. 45 A section immediately posterior to the optic stalk in an embryo 

 without a circulation when seventy-two hours old; Experiment A, 1913. A nest 

 of peculiar finely granular cells lies in the mesenchyme which contains many 

 dividing cells; Br, brain; Opv, optic vesicle. 



Fig. 46 Cells from a four-day embryo; Experiment 11, 1912; Men, mesen- 

 chyme nucleus; small leucocyte-like cells are grouped in the neighborhood of 

 chromatophores. 



Fig. 47 A binuclear leucocyte from a sixteen-day-old embryo. 



Fig. 48 Section through one of the dorsal aortae in a four-day embryo; 

 Experiment 11, Embryo 6; embryonic leucocytes in the mesenchyme. 



