574 CHARLES R. STOCKARD 



to change from the globular to the elipsoidal shape, the accumu- 

 lation of haemoglobin takes place and the cells begin to show a 

 typical straw color. 



It may then actually be seen in the living that the early or 

 primitive erythroblast is really a more or less globular amoeboid 

 cell without haemoglobin and resembling more closely a lympho- 

 cyte or early leucocyte than a fully formed erythrocyte. This 

 is probably true of the early stages in many cases of cytomor- 

 phosis, yet the globular amoeboid cells of the yolk islands are not 

 indifferent 'primitive blood cells' in the sense Maximow ('09) has 

 concluded, but they are definitely future erythrocytes. 



This point is established by a study of these cell groups in 

 the normal as well as in embryos without a circulation of the 

 blood. In the latter individuals the islands arise by the forma- 

 tion of local aggregations of the early wandering cells in an 

 exactly similar manner to that described for the normal em- 

 bryo. In fact, the observer cannot distmguish between the 

 two specimens in many cases, yet one fails to establish a circu- 

 lation and the islands are thus enabled to retain their positions 

 on the yolk-sac. 



The constituent cells of such a permanent island may be 

 observed from time to time or continuously, and will be found to 

 pass through changes exactly identical with those which take 

 place in the island cells that become swept into the blood stream 

 of the normal embryo. They are for a few days globular in 

 shape, but capable of slightly changing their form and sending 

 out short pseudopod-like projections. When the embryos are 

 five or six days old the cells in these islands then become flattened 

 ellipsoidal corpuscles and attain a haemoglobin content exactly- 

 as in the normal embrj^o. The blood islands now appear bright 

 red in color and are quite conspicuous on the yolk-sac where they 

 permanently remain. The globular colorless cells are thus seen 

 to differentiate directly into the tj^pical ictheoid erythrocyte. 



From a study of the living embryos alone one could not, of 

 course, be certain that all of the cells of these islands had differ- 

 entiated into red blood corpuscles. However in the previous 

 study of the non-circulating embryos, I have examined a large 

 number of yolk islands completely and have never seen any 



