130 INTRODUCTION. 



FIG. 60. THREE STAGES IN THE DEVELOPMENT OF SAGITTA. (A and C after 

 Biitschli and B after Kowalevsky.) The three embryos are represented in the same 

 positions. 



A. Represents the gastrula stage. 



B. Represents a succeeding stage in which the primitive archenteron is com- 

 mencing to be divided into three parts, the two lateral of which are destined to form 

 the mesoblast. 



C. Represents a later stage in which the mouth involution (i) has become con- 

 tinuous with alimentary tract, and the blastopore has become closed. 



m. mouth ; al. alimentary canal; ae. archenteron ; /'/. /. blastopore ; pv. perivis- 

 ceral cavity ; sp. splanchnic mesoblast ; so. somatic mesoblast ; ge. generative organs. 



the stomodaeum and proctodaeum, derived from epiblastic 

 invaginations. They give rise respectively to the oral and anal 

 extremities of the alimentary tract. 



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