MY RI APOD A 411 



ment. Cleavage nuclei appear at the surface of the egg and by further 

 division form a blastoderm of flattened cells. Thus cleavage is purely 

 superficial. Connected with the blastoderm cells by protoplasmic 

 processes are cells that have remained behind in the yolk. Of these, 

 some will form the somatic and splanchnic mesoderm; others will form 

 the mid-gut epithehum; and still others, after the mid-gut epithelium is 

 formed, give rise to other mesoderm cells to form various muscles and 

 the circulatory system. The amoeboid-Uke cells which are to enter into 

 formation of the mesoderm approach the future ventral surface of the 

 egg where they gather to form a longitudinal strip along the mid-ventral 

 line. Here the}^ become more compact with rounded nuclei. These 

 cells by subsequent division together with cells derived from the ectoderm 

 ectad of them soon form a longitudinal strip several layers in thickness. 

 This strip later spreads out and becomes segmented. The somites are 

 solid at first, but later a cavity appears in them. Early on the ninth 

 day the stomodaeal invagination is evident, with the proctodaeum 

 appearing soon after. The entoderm cells are still scattered within the 

 yolk, but they gradually become collected on the median line just entad 

 of the mesoderm. The stomodaeum and proctodaeum become more 

 deeply invaginated, extending a considerable distance into the yolk; 

 and at the same time the entoderm cells, loosely connected, begin to form 

 the mid-gut epithelium, arranging themselves around a central lumen. 

 At the end of the ninth day the embryo is long and oval in shape; the 

 mesoderm is divided into eight segments, the first of which will form the 

 future head; and the mid-gut epithehum is almost fully formed. 



The tenth day brings about the completion of the ectodermal seg- 

 mentation, the formation of the nervous system, and the formation of 

 the ventral flexure, the mesodermal somites acquiring cavities with the 

 embryo assuming its definitive shape. The ventral flexure is first 

 formed by a deepening of the transverse furrow which forms the division 

 between the seventh and eighth segments (Fig. 369); the head and tail 

 ends nearly touch. The eighth segment is considerably longer than any 

 of the others except the head. Even on the eleventh and twelfth days, 

 the tissues of this segment are still imperfectly differentiated. At a 

 later period of development the anal segment is constricted off from the 

 eighth segment, and from its anterior part the future segments formed in 

 later life are developed. Just before the first appearance of the ventral 

 flexure when the body segments are fully formed, the embryo develops 

 a cuticular envelope over the whole surface which separates from the 

 body. 



Of the nervous system the bilobed cerebral parts are formed first, 

 followed by nerve cords which develop progressively backward. The 

 cords are at first widely separated but connected by a thin median por- 



