138 VICTOR E. EMMEL 



2. CYTOPLASM'lC PEOCESSES 



Not infrequently one encounters in the circulation of the pig 

 embryo a red blood corpuscle which instead of having the usual 

 smooth contour, is characterized by the presence of a cytoplasmic 

 process projecting from its surface. Typically this process tapers 

 to a somewhat blunt point (fig. 7). Occasionally the tip of the 

 process may be partly bifurcated. A few cells were found having 

 more than one process. These structures occur in the non-nucleat- 

 ed erythrocytes as well as in the erythroblasts (fig. 8). Since 

 these processes are present in the fresh as well as in the most 

 carefully fixed vessels, and the corpuscles having them appear 

 otherwise perfectly normal in both their structure and stain, 

 they are evidently to be regarded as structures normally present 

 in the embryonic circulation. Engel ('99) also observed these 

 cytoplasmic processes in fixed preparatioais of the blood of the 

 pig embryo, and regarded them as normal structure : 



Wie im frischen Zustande zeigten auch einige Metrocyten I Gen- 

 eration ein, selbst zwei spitz zulaufende Enden, von denen wohl nicht 

 erst betont zu werden braucht, dass sie als Kunstproduct nicht anzu- 

 sprechen sind. Eine Erklarung fiir diese Zellform muss ich mir so lange 

 versagen, als es mir noch nicht gelungen ist, noch jiingere Schweineem- 

 bryonen zu untersuchen. Es mlisste sich dann zeigen, ob die durch ihre 

 hamoglobinhaltigen Protoplasmafortsatze, breiten Spindelzellen ahn- 

 lichen Blutkorperchen, moglicherweise fixe, hamoglobinhaltige embryon- 

 ale Endothelzellen gewesen sind, bevor sie die runde Form der freien 

 Blutkorperchen erlangten (p. 39). 



The question at once arises as to the origin of these structures. 

 Engel's suggestion that they may represent remnants of the orig- 

 inal endothelial cells, which had not yet disappeared in the 

 course of differentiation, does not appear tenable for two reasons. 

 First, these processes appear to occur more frequently in 25 to 

 35 mm. embryos than in younger specimens, and second, what 

 appear to be identically similar structures may also be found in 

 the non-nucleated corpuscles. An explanation which has sug- 

 gested itself in the course of the present study is that the primary 

 origin of these cytoplasmic structures is to be traced back to 

 processes of cell division. As is well known, in the young mam- 

 malian embryo mitotic division of the young erythroblast may 



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