CHAPTER XXV 



EMBRYOLOGY 1 



Hitherto we have been concerned almost wholly with the 

 anatomy and physiology of adult animals. We 

 Eariystages. must now gi ve some attention to the process by 

 which the adult arises from the fertilised egg. 

 For this purpose we shall study first the development of 

 the lancelet, which is relatively simple and easy to follow 

 owing to the fact that the protoplasm of the ovum is not 

 hampered with a large amount of yolk. The egg is about 

 o - i mm. in diameter, covered with a slight vitelline membrane. 

 Only the second polar body is found on it after it has been laid. 

 The first cleavage (Fig. 358, 2) is vertical and forms two equal 

 blastomeres (p. 109). The second is also vertical, at right 

 angles to the first, the third nearly equatorial, dividing each 

 blastomere into a rather smaller upper half and a rather larger 

 lower half. The blastomeres do not meet in the middle, so 

 that at this stage they form a ring. By repeated divisions, 

 vertical and horizontal, a hollow sphere or blastula arises, 

 whose cavity or blastocosle is walled by a single layer of 

 cells, rather larger and with more yolk granules on one side 

 (Fig. 359, 7). This side becomes first flattened and then 

 tucked or invaginated into the blastoccele, till the latter is 

 practically lost and a two-layered cup or gastrula has arisen. 

 In spite of the difference in shape we may compare this with 

 the body of a Hydra. The outer layer of the body is the 

 ectoderm or epiblast, and the inner layer the endoderm or 

 hypoblast, the hollow of the cup is the primitive gut or 

 arckenteron, and the mouth, as yet wide, is called the blasto- 

 pore. The gastrula now lengthens, owing to the growth of the 



1 In reading this chapter, the student should refer constantly to the 

 figures. 



