CHAPTER V 



EARLY DEVELOPMENT OF THE EMBRYO 



In the preceding chapter the cleavage of the egg has been 

 described to the period when the blastopore is about to appear 

 on the surface. During the subsequent development the cells 

 continue to divide, so that at no time can the cleavage or the 

 cell-division be said to cease. At each successive stage the 

 number of cells is greater than in the preceding stage. This 

 statement does not imply, however, that the formation of each 

 new structure is introduced by new cell-divisions in the region 

 where the change is about to begin, because many changes take 

 place in regions where cell-division is not more rapid than else- 

 where. 



The spherical form of the " egg " or young embryo is soon 

 lost. In the present chapter we shall follow the changes that 

 can be seen taking place on the exterior of the living embryo ; 

 and in the following chapter we shall attempt to make out the 

 movements of cells and groups of cells that take place in the 

 interior of the embryo during this period. 



The Blastopore 



On that side of the egg where the smaller cells are found, a 

 short horizontal line of pigment 1 appears amongst the white 

 cells below the equator of the egg (Fig. 12, I). This line marks 

 the beginning of the archenteron, and the cells bounding the 

 upper or darker side of the pigment-line form the dorsal lip of 

 the blastopore. The dorsal lip becomes crescentic in outline, 

 with the concavity of the crescent turned toward the white 

 hemisphere (Fig. 19, I, II). If the living egg be watched, it 



1 There is a great deal of variation at first in the shape of the blastopore. 



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