176 VICTOR E. EMMEL 



present evidence for their origin by cytoplasmic constriction may, 

 therefore, be justifiably introduced for consideration. 



In the foregoing review of the evidence for nuclear extrusion it 

 becomes evident that conclusions supporting this view are based 

 primarily on data drawn more largely from observations of the 

 vascular system of the adult and on the embryo approaching 

 term. Since the present study deals only with embryos up to 40 

 mm. in length, a discussion of the extensive data and literature 

 for nuclear extrusion in the bone marrow of older embryos and 

 the adult on the basis of Ihe present observations is not warranted. 

 However, it is of interest to note that the erythroblasts of the 

 bone marrow of the adult, from which the data under considera- 

 tion have been derived, are chiefly of the smaller type or so-called 

 normoblasts. Consequently, if the conclusion for the origin of 

 plastids in the embryo by cytoplasmic constriction be correct, 

 it remains to be determined whether possibly the apparent nuclear 

 extrusion described for the bone marrow of the adult may not 

 be fundamentally a process of cytoplasmic constriction in which, 

 on account of the smaller size of the erythroblast, stages in this 

 process may present the appearance of an extrusion of the nucleus, 

 whereas in the case of the large erythroblasts of the embryo, with 

 a greater quantity of cytoplasm, the real character of the process 

 is more clearly evident.^ 



Before proceeding further in discussion, a digression is desirable 

 concerning the erythroblasts of the early mammilian embryo. 

 Erlich ('80) and a number of subsequent investigators have sought 

 to make a distinction between the larger nucleated erythrocytes 

 or megaloblasts of the embryo and the smaller nucleated cor- 

 puscles or normoblasts of the adult, and have maintained that 

 they are genetically distinct in both their origin and mode of 

 differentiation. In the present study of the pig embryo no substan- 



^•In some of the figures which various investigators have given as evidence 

 for nuclear extrusion, emphasis has been placed upon the compressed or elongated 

 form of the nucleus as indicative of its being forcibly squeezed or pressed out of 

 the cell body. There appears, however, to be some ground for the question 

 whether the possibility has been eliminated that some of .these may be instances 

 of merely profile views of nuclei already, flattened in the course of normal differ- 

 entiation previous to the formation of the plastids, as has been shown to occur in 

 the pig embryo (cf. figures 22, 45, and pp. 135, 142.) 



