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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



here and there a separation of one part from another. The processes 

 of development might be imitated with a piece of soft chiy or model- 

 ing wax, manipulated without adding anything and without taking 

 anything away, pushing it in there, pulling it out here, until the 

 form of the new creature is at last produced. The insect embryo 

 ordinarily does not grow in size, except as it utilizes the yolk and 

 replaces the latter with its own tissues. 



Th 



>Ab 



Fig. 14. — Two stages of the Insect embryo, drawn dlagrammatically, 



probably representing two periods in the evolutionary history of 



insects 



A, early embryo consisting of embryonic head, or procephalon (Pre), 

 formed of three segments, and of a body (Bdy) with as many as 18 

 segments, each but the last with a pair of ventral appendages (Apn). 

 B, a later stage, showing segment grouping of adult insect: head (H) 

 of six segments; thorax (Th) of three segments; abdomen (AB) of 

 at most twelve segments. 



Ah, abdomen ; Ant, antenna ; 2Ant, rudimentary second antenna ; 

 Bdy, body ; Cer, cercus ; E, compound eye ; H, head, of adult com- 

 position ; L, legs ; Lin, labrum ; Md, mandible ; IMx, first maxilla ; 

 iMx, second maxilla ; Pre, primitive embryonic head, or procephalon ; 

 Proc, opening of protodeum (the anus) ; Stom, opening of stomodeum 

 (the mouth) ; Th, thorax. 



SEGMENTATION 



Very early in tlie development of the insect embryo the two- 

 layered germ band becomes marked by crosswise impressions, the 

 lines appearing near the anterior end first and increasing in number 

 backward. The continuous germ band thus becomes divided into 

 a series of segments separated from one another by thinner inter- 



