4 OUTLINES OF CHORDATE DEVELOPMENT 



the larva is about 3.5 mm. long and has reached a condition 

 known as the "critical stage/' 



IV. ADOLESCENT PERIOD. From the critical stage to sexual 

 maturity. This is marked by histological differentiation and 

 the gradual appearance of adult characteristics. Germ cells 

 are first brought to maturity in specimens about 2 cm. in length, 

 the age of which is doubtful. Growth to full size probably in- 

 volves several years and covers several spawning periods. 



I. THE GERM CELLS AND THEIR PRODUCTION 



We may first describe the germ cells as they are extruded 

 from the body. 



The eggs of Amphioxus are spherical and only 0.10 to 0.13 

 mm. in diameter. Among the Chordates smaller eggs than 

 these are found only in some Mammals. The egg (Fig. 1) is 

 surrounded by two membranes, a thin outer, or vitelline mem- 

 brane, which at this time is usually separated by a space 

 from the surface of the egg, and a thick inner, or perivitelline 

 membrane, which at the time of laying is more or less fluid 

 and closely adherent to the surface of the egg. The egg is 

 richly supplied with deutoplasm in the form of very numerous 

 small granules. These are not uniformly distributed through 

 the cytoplasm for there are two definite regions comparatively 

 free from deutoplasm, namely, a narrow superficial layer, and an 

 irregular conical region toward one side of the egg. Strictly 

 speaking the egg of Amphioxus is thus telolecithal, but the deu- 

 toplasmic center is not toward the vegetal pole of the egg but 

 toward one side, below the equator. The precise location of this 

 yolk-free area is important for it establishes in the egg a definite 

 bilateral symmetry which is also that of the future embryo. 

 The animal pole is marked by the apex of this clear area and by 

 the point at which the first polar body is given off. The first 

 polar body has already been formed at the time the egg is laid, 

 but since it lies outside the vitelline membrane it is ordinarily 

 lost. At this time the animal pole is marked, however, by 

 the position of the egg nucleus which is just in the equatorial 



