CHAPTER XIII 



DEVELOPMENT OF THE LANCELET : EARLY DEVELOPMENT 

 OF OTHER TYPES, INCLUDING THE CHICK AND RABBIT : 

 FORMATION OF THE CHIEF ORGANS OF VERTEBRATES, 

 AND OF THE AMNION, ALLANTOIS, AND PLACENTA. 



THE oosperm of Amphioxus (Fig. 152, A) undergoes 

 binary fission (B) , each of the two resulting cells dividing 

 again into two (C). This process is continued until 

 a globular mass of cells or blastomeres is produced 

 by the repeated division of the one cell which forms 

 the starting point of the series. Owing to there being 

 rather more yolk at the lower than at the upper pole, 

 the lower cells are slightly larger than the upper, 

 so that the segmentation is not quite equal (D, E). 

 The embryo has now arrived at the polyplast or morula 

 stage, and sections show that it is hollow, the blasto- 

 meres being arranged in a single layer around a central 

 segmentation-cavity or blastoccele (compare p. 200) : such 

 a hollow morula is known as a blastula (F, G). The 

 lower side of the blastula then becomes tucked in, or 

 invaginated, the result being that the single-layered 

 sphere is converted into a double-layered cup (H, I). 

 This process can be sufficiently well imitated by pushing 

 in one pole of a hollow india-rubber ball with the finger. 

 The resulting embryonic stage is known as the gastrula : 



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