NO. 3 EMBRYOLOGY OF FLEAS KESSEL 6l 



The trophonuclei have two origins. The primary trophonuclei are 

 formed from cleavage nuclei which remain within the vitellus when 

 the first blastema stage is produced. The secondary trophonuclei are 

 derived from cells which return into the vitellus from the blastoderm 

 or mesoderm of the germ band. Secondary yolk cleavage does not 

 occur. 



Posterior polar granules are not evident. The germ cells are poly- 

 nuclear in origin. Their number varies from 5 to 12 in different eggs. 

 The first protoplasmic pockets appear at the posterior pole during the 

 first blastema substage. The germ cells produced thereby complete 

 their constriction during the second blastema substage. More lateral 

 ones may be somewhat delayed. The germ cells reenter the egg before 

 blastulation. 



The mesoderm originates by three methods. In the anterior third 

 of the germ band this is by simple emigration of cells from the blasto- 

 derm. In the middle region a shallow median longitudinal groove 

 assists in the process. Along the sharp bend of the germ band, where 

 the embryonic rudiment is involuted into the vitellus, the groove is 

 transformed into an invaginated mesodermal tube. In the short region 

 between the section of tube formation and the posterior mesenteron 

 rudiment, a simple groove is again produced. Along the short length 

 of the germ band which is carried around the terminus of the in- 

 vaginating proctodaeum, the mesoderm is again formed by simple 

 emigration. 



The germ band is partially superficial and partially involuted at 

 the time of mesodermal differentiation. The amnion and serosa are 

 formed by the overgrowth and fusion of the amnio-serosal folds, aided 

 in the posterior region by the involution process. Both embryonic 

 membranes disappear before eclosion, the serosa first and the amnion 

 later, each rupturing and forming a dorsal organ. The dorsal organs 

 are absorbed by the vitellus. 



The growth and movements of the germ band or embryo are as 

 follows. The formation of the ventral plate occurs on the first day of 

 development. Involution of the posterior portion of the embryonic 

 rudiment takes place on the second day. Withdrawal of the involuted 

 section of the germ band follows during the first part of the third 

 day. The anterior and posterior extremities lie close together on the 

 dorsal surface at this stage. On the fourth day the embryo shortens 

 toward the poles of the egg. On the fifth day it flexes ventrally and 

 begins to lengthen. This elongation is continued during the sixth or 

 final day of development so that the extremities coil upon themselves, 

 one on either side of the middle portion of the body. 



