Chsetoptenis Yariopedatns, 509 



Fertilization. 



Mead ('98) made the observation that the "Eipe eggs may be 

 carried in the body-cavity several days before they are laid. Dur- 

 ing this time neither centrosome nor aster can be distinguished, 

 though the reticulum is unusually distinct. In a few minutes after 

 the eggs have been deposited in sea-water, however, a large number 

 of asters are developed by rearrangement of the cytoplasmic net- 

 work." 



The eggs in the nephridia are to be found in the same stages as 

 those in the body cavity and at no time do they show any sign of 

 an entering spermatozoon or of a male pronucleus. Among the eggs 

 discharged from the animal I found young ones whose germinal vesi- 

 cles were still intact. The germinal vesicle of these young ova 

 remained intact until the eggs were extruded into the sea-water, 

 then it broke down and the first maturation spindle was formed. 

 These eggs and the ones whose maturation occurred in the body cavity 

 and nephridia may remain in this condition for an indefinite period 

 or until fertilization occurs. The entrance of the spermatozoon, 

 which is of the ordinary tailed form, causes the rapid formation 

 of the polar bodies and female pronucleus, while the male pronucleus 

 is still quite minute and at the distant portion of the egg. The 

 egg at first is spherical and uniformly opaque, but its animal pole 

 becomes flattened and clear at the time that the first polar bodies 

 are formed. 



Segmentation^ and Formation of the Trochophore. 



The segmentation was studied and correctly described by E. B. 

 Wilson ('83) and Mead ('97). Its external appearance is, briefly, 

 as follows. A few minutes after fertilization a delicate membrane 

 from the oosperm and the polar bodies are extruded, at intervals of 

 a quarter of an hour, from the clear end of the elongated egg. 



In its first cleavage the oosperm is divided into two cells of unequal 

 size, by a plane passing through the polar bodies. A large "yolk- 

 lobe" is formed nearly opposite the polar bodies during the internal 

 changes which accompany the first cleavage furrow. The second 

 cleavage also is meridional and at right angles to the first. The 



