58 OUTLINES OF CHORDATE DEVELOPMENT 



genital cells and the atrial epithelium, enlarges above and below 

 the region of the original stalk of attachment, which, by the 

 general growth of the gonad, is pushed from the myotomal to 

 the atrial surface. This cavity is now known as the secondary 

 gonadial (ovarian or testicular) cavity (Fig. 21, C). In the 

 outer wall of this, just above and below the stalk of attachment, 

 two specialized thickenings develop; these are the cicatrices. 

 As the gonadial cells multiply and enlarge they crowd upon 

 these cavities and nearly obliterate them, particularly those 

 toward the myotomal surface, but the essentially epithelial 

 arrangement is not lost, and as the ova develop, as described 

 at the beginning of this chapter, their polarity has a definite 

 relation to their position in the epithelium, such that the animal 

 pole is toward the free surface. When the germ cells are fully 

 developed the inner and outer gonadial envelopes contain 

 muscle fibers, the contractions of which appear to assist in 

 rupturing the visceral layer of the gonocoel in the region of 

 the secondary gonadial cavity, and force the germ cells into 

 this cavity, where they remain for some time before extrusion. 

 Sexual differentiation within the gonad appears in specimens 

 about 18 mm. in length. 



When the fully formed germ cells, ready for laying, have 

 accumulated in the secondary gonadial cavity, strong contrac- 

 tions of the body wall and ventral musculature rupture the 

 outer membrane in the region of the cicatrices, and they are 

 forced into the atrial cavity, whence they are carried to 

 the outside through the atriopore by the respiratory current, 

 aided by continued muscular contraction. Individuals of the 

 common species of Amphioxus apparently first produce mature 

 germ cells, that is, become adult, when they reach a length of 

 about 2 cm.; the age of such specimens is unknown. 



REFERENCES TO LITERATURE 



CHAPTER I 



The literature lists here and at the end of Chapters III, V and VI, 

 include only a few of the more important or more recent titles. Refer- 

 ences to the sources of borrowed figures are included, and a few works 



