240 John H. Gekoüld, 



for egg-laying, but I have obtained small deposits of eggs at Wood's 

 Hole in August and early in September. 



Egg-laying usually occurs between 8 p. m. and 4 a. m. General 

 muscular relaxation and expansion of the body are produced by 

 keeping the animals in darkness during the day. This tends to 

 bring about an earlier segregation of the mature germ cells into 

 the nephridium, and earlier egg-laying, than usual, but these processes 

 have an established rhythm, that is largely independent of present 

 external conditions. 



A few hours before egg-laying occurs, the nephridia become 

 distended with a transparent fluid, into which the germ cells are 

 swept from the coelom by the action of the cilia of the nephrostome. 

 The ova in the nephridium have the first polar spindle in meta- 

 phase. 



Spermatozoa are ejected in cloud-like jets through the nephridio- 

 pores. Contact of the spermatozoa with the skin stimulates the 

 mature females in the vicinity, and hastens the deposit of eggs, 

 which are forcibly ejected in showers. Spermatozoa within the 

 coelomic fluid are quiescent, but become motile as soon as they are 

 extruded into the sea water, which stimulates them to activity. 



Maturation and Fertilization. The bullet-shaped head 

 of the spermatozoon, upon entrance into the cytoplasm, rotates 180 " 

 about its transverse axis, and a small astrosphere, which contains a 

 minute centrosome, appears at its base in the region of the middle 

 piece. At this time, the protoplasmic processes which surround the 

 egg disappear, and are probably retracted into the cytoplasm. The 

 astrosphere precedes the sperm nuclus to the centre of the egg, 

 while the first polar body is being formed. A deeply-staining rod 

 connects the astrosphere with the nucleus. Having reached the 

 centre of the egg, both sperm nucleus and its centrosome increase 

 greatly in size, but neither of them divides. 



The spindle of the first maturation division has ten chromosomes 

 (the reduced number), which have the shape of elongated rings or 

 rods, which lie parallel to its long axis. These rings break apart 

 in the middle by a reduction division, each forming two U-shaped 

 chromosomes, which are characteristic of the second maturation 

 spindle. In the latter, the U-shaped chromosomes lie transversely 

 to the length of the spindle, with the curve of the U touching its 

 equator and with the points directed outward. The chromosomes 

 then become divided, each at the curve of the U, where the spindle 



