ERYTHROBLASTS IN THE PIG EMBRYO 181 



although he makes the reservation ''Mais nous ne voulons pas 

 aujourd' hui insister sur ce rapprochement, car nous ne pouvons 

 encore nous prononcer sur la veritable valeur de ce phenomene, 

 dans Fobject que nous etudions" (p. 485). Engel ('94) records 

 the occurrence of non-nucleated red blood corpuscles in the early 

 chick embryo. Engel ascribed their origin to the pinching off 

 of portions of the cytoplasm of the nucleated erythrocytes, a con- 

 clusion which he has reaffirmed in 1906 as follows: ''In viellen 

 Fallen, sowhol beim Hlinchen als beim Frosch, hing der kernhal- 

 tige Teil durch eine protoplasmatische Verbindung noch mit den 

 kernlosen Telle zusammen. Es bedarf wohl kaum besonderer 

 Erwahnung, dass in diesen Fallen von einem Praparationsfehler 

 oder einem Kunstprodukt keine Rede sein kann" (p. 146). Dan- 

 tschakoff ('08), while not attributing any especial significance to 

 the phenomenon other than its being probably degenerative in 

 in character, also notes the occurrence of non-nucleated red cor- 

 puscles in the embryo chick. 



Though granting the necessity for further investigation, never- 

 theless, these results lend increased interest to the observations 

 for mammals, for if these non-nucleated corpuscles occurring occa- 

 sionally in lower vertebrates are to be regarded as precursors of 

 the respiratory elements which were to predominate in the circu- 

 latory system of the adult mammal, their mode of origin in the 

 lower vertebrates may throw light on the nature of the process in 

 the higher forms. If the mammalian embryo is in many respects 

 morphologically and phylogenetically comparable to ancestral 

 vertebrates, the evidence for the occurrence of processes of cyto- 

 plasmic constriction in the origin of hemoglobin-containing plas- 

 tids common to both groups of animals may be a matter of more 

 than passing significance. 



3. THE ROLE OF CYTOPLASMIC CONSTRICTION IN THE ORIGIN OP 



OTHER NON-NUCLEATED MORPHOLOGICAL ELEMENTS OF THE 



MAMMALIAN BLOOD 



During the last few years evidence has accumulated indicating 

 that the constriction off of non-nucleated cytoplasmic portions 

 of cells may play an important role in the formation of several 



