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ANNUAL REPORT SMITHSONIAN INSTITUTION, "1925 



protoplasm that divides, the rest of the egg remaining intact 

 (fig. 10 A). As the newly formed nuclei redivide, the residting 

 nuclei scatter through the yolk (B) and "migrate outward in 

 all directions until they come into the i^rotoplasmic layer at the 

 surface. Here they eventually become evenly arranged in a single 

 layer (C), and then the containing protoplasm condenses about each 

 luiclcus (D), resulting in a layer of small nucleated cells envelop- 

 ing the yolk (E). This cell layer, having now the form of the 

 eg^fi:, is the hlastoderm (D, P], Bl). Eggs that divide by the entire 

 method arrive at the blastoderm stage simply by multiplying until 

 they form a hollow ball of cells. This is a simpler procedure, but 

 one that is practicable only with eggs that are not heavily loaded 

 with 3'olk. 



KiG. 10. — Formation of the blastoderm 

 A, development begins with the division of the egg nucUnis into several 

 cleavage nuclei (JVw) within the yollc (V). B, the multiplying nuclei migrate 

 outward into the cortical layer of protoplasm (CL) ; several at the posterior 

 pole of the egg become the germ cells (OCls). C, the nuclei in the cortical 

 protoplasm soon form a definite layer at the surface of the egg ; a few nuclei 

 remain in the yolk ; the germ cells increase in number. D, the cortical proto- 

 plasm condenses about each surface nucleus to form a layer of cells, the blasto- 

 derm (Bl), surrounding the yollf beneath the vitelline membrane (Vit). E, 

 the blastoderm in surface view. 



FORMATION OF THE GERIM CELLS 



The cells of the blastoderm are in general all alike, but in many 

 insects a group of cells appears at the posterior end of the .egg 

 (fig. 10 B, C, GCls) at the time the blastoderm is forming which 

 remain distinct from the true blastoderm cells. These cells like- 

 w^ise are immediate descendents of the egg nucleus, but their 

 destiny is quite different from that of the other cells — ih&y are 

 the primitive germ cells of the future individual whose body is 

 to be formed by the blastoderm cells. Here we have, then, differ- 

 entiated at this early stage, two sets of cells, one of which will 

 form the body cells of the generation now developing, while the 

 other will form the body cells and the germ cells of the succeed- 

 ing generation. This relation is easily seen in the diagram, Eig- 



