312 Papers from the Department of Marine Biology. 



DESCRIPTION OF ILLUSTRATIONS. 



All the illustrations, unless otherwise specified, are made from the 5 mm. embryo; the magni- 

 fication is 1,600 diameters, which is reduced one-third in reproduction. 



Fig. 1. — Hemoblasts from yolk-sac vessels: a, typical spheroidal type, with basophilic cytoplasm 

 and deep-staining nucleus; b, similar cell in amoeboid condition; c, slightly more differen- 

 tiated type, with lighter-staining nucleus and cytoplasm (young erj-throblast) ; d, binucleated 

 hemoblast differentiated into a young erythroblast; c, giant cell (megakaryocyte) with deeply 

 basophilic cytoplasm and irregular, horseshoe-shaped, deeply staining nucleus. 



Fig. 2. — Portion of yolk-sac wall, including one large blood-channel filUed with erythrocytes. 

 The mesothelium is shown on the left, the entoderm on the right; a to d designate successive 

 stages in the metamorphosis of an endothehal cell into an erythrocyte. The cell at b has all 

 the nuclear and cytoplasmic marks of a hemoblast, but it is still continuous with the endo- 

 thelial wall, c, a very large hemoblast, also still continuous with the endothelium of the 

 vessel. 



Fig. 3. — Hemoblast just after separation from the endothelium. 



Fig. 4. — A binucleated elongated hemoblast, still continuous with the endothelium and about 

 to divide amitotically into two hemoblasts. The three large cells at the left are entodermal. 



Fig. 5. — Cell-cluster arising in latero-ventral wall of mesonephric portion of aorta through pro- 

 liferation and differentiation of the endothelium. The peripheral cells have hemoblast 

 characteristics. 6 mm. embryo. 



Fig. 6. — A similar small cluster from the ventro-medial portion of the aorta. Proximally the 

 cluster is in syncytial continuity with the endothelium. 



Figs. 7, 8, 9, 10. — Hemoblasts differentiating and separating from the lateral wall of the aorta. 



Fig. 11. — A string of three hemoblasts, the cytoplasm of which is still in continuity, and which 

 have differentiated from the endothelium of the latero-ventral wall of the aorta. The prox- 

 imal hemoblast is still in continuity with the endothelium. 6 mm. embryo. 



Fig. 12. — A string of three hemoblasts (a) jutting from the endothelial wall into the lumen of 

 the aorta. Within the lumen are shown five erythrocytes; b, young erythroblast (megalo- 

 blast) with finely granular slightly acidophilic cytoplasm ; c, normoblasts; d, adult erythrocyte. 

 6 mm. embryo. 



Fig. 13. — Binucleated hemoblast differentiating from the wall of a pericerebral blood-channel. 

 The proximal nucleus and its enveloping cytoplasm have endothelial characteristics, and 

 are continuous with the endothelium, which is continuous above with the mesenchyma. 

 The distal nucleus and its enveloping cytoplasm have hemoblast characteristics. 



Fig. 14. — Pericerebral capillary with one ei"ythrocyte. This condition may be conceived to 

 have arisen from a binucleated cell as in figure 13, in which one nucleus and the peripheral 

 portion of the cytoplasm differentiated into endothelium, the other into an erythroblast. 



Fig. 15. — Hemoblast differentiating from the endothelial wall of a pericerebral vascular space. 



Fig. 16. — Pericerebral blood-vessel in which an endothelial cell is dividing internally by mitosis. 



Fig. 17. — Pericerebral blood-vessel upon which an endothelial (mesenchymal) cell is dividing 

 externally by mitosis. 



Fig. 18. — Pericerebral blood-vessel in cross-section (containing an erythrocj^te) from one point 

 of which is sprouting a lateral vascular twig. The sprouting endothelial cell is at a late 

 anaphase of division. 6 mm. embryo. 



Fig. 19. — Cell-cluster of the atrial visceral pericardium. These clusters are similar to those of 

 the aorta, the constituent cells including some with hemoblast characteristics. 



Fig. 20. — Hemoblast-like cell separating from the mesothelial layer of the aortic bulb. 



Fig. 21. — Free cell-cluster from the pericardial cavity. 



Fig. 22. — Portion of visceral layer of capsule of mesonepheric glomerulus, showing cells in process 

 of separation. 



