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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



former constitute the ectoderm (epiblast), the latter the endoderm 

 (hypoblast). 



G-astrulation (Fig. 40), by which the single-walled blastula is 

 converted into the double-walled gastrnla, follows cleavage and 

 occupies about six hours. The endoderm cells undergo a gradual 

 inpushing or invagination (emboly, embolic invagination) until 

 the blastocoele is obliterated, and the embryo has become cup- 

 shaped, with a central digestive cavity (archenteron) opening by 

 a wide blastopore. The endoderm cells now line the archenteron 

 and are covered by the ectoderm, each cell of which develops a 

 flagellum. The cup-shaped gastrula soon assumes an ovoid shape 

 with a flattened dorsal and a convex ventral surface. Meanwhile 



Fig. 40. BLASTULA AND GASTRULA OF AMPHIOXUS (from Clans, after 

 Hatschek). A, In optical section. A, Blastula with flattened lower 

 pole of larger cells. B, Commencing invagination. C, Gastrulation 

 completed ; the blastopore is still widely open, and one of the ineso- 

 dermic teloblasts is seen at its ventral lip. The flagella of the ecto- 

 derm cells are not represented. 



the blastopore has narrowed to a small rounded aperture, and is 

 now situated at the posterior end towards the dorsal surface. Two 

 endoderm cells on the ventral side of the blastopore are distinguished 

 by their relatively large size. These are the mesodermic telo- 

 blasts, which subsequently originate a part of the mesoderm. 



The completed gastrula escapes from the vitelline membrane and 

 swims freely, front end first, by means of the ectodermal flagella. 



Origin of the Mesoderm, Ccelom, Muscles, Notochord, and 

 Nervous System (Figs. 41 and 42). The foundations of all 

 these are simultaneously laid during the twenty-four hours which 

 succeed hatching. 



The mesoderm in the anterior part of the body is constituted 

 by the walls of myoccelomic pouches which successively grow out 



