The maturation and fertilization of the egg of Cerebratulus. 433 



The third cleavage, which is distinctly right-handed, shows the 

 first differentiation of the cells in regard to size, the upper four 

 or those lying next to the polar bodies, being percep- 

 tibly larger than the lower four. 



The eggs of Micrura caeca ^) are very similar to those of 

 Cerebratulus leidyi, and are to be obtained at about the same time 

 of the year. They are slightly larger than those of the latter species, 

 measuring a trifle less than 0,1 mm in diameter. They develop a little 

 more slowly and reach the 2-celled stage in about 1 hour and 

 20 minutes. The embryos may readily be kept alive for twelve or 

 more days. As in C. leidyi the upper four cells of the eight-celled 

 stage are distinctly, though slightly, larger than the lower four. 



Lineus socialis ^) matures its genital products in mid-winter. The 

 eggs are sometimes deposited in confinement, and in such cases they 

 are found on the bottom of the jar in irregular masses surrounded 

 by a little mucus. The eggs measure 0,093—0,1 mm in diameter. They 

 are much more opaque than those of C. leidyi^ and therefore not 

 nearly so well adopted for the study of the living egg. 



The eggs of Cerebratulus lacteus^) are to be obtained in abund- 

 ance during the mouth of March, and sometimes in April. Although 

 the adult animals probably have the greatest bulk of any known 

 Nemerteans, yet their eggs are but little larger than those of the 

 species mentioned above. A single specimen yields an enormous 

 number of eggs, but they are so opaque that the processes of maturation 

 and fertilization cannot so easily be studied while living as those of 

 some of the other species. 



1) Vereill, in: Trans. Connecticut Acad., V. 8, 1892. This species 

 is sometimes associated with Cerebratulus leidyi, but is more commonly 

 found under stones, and in more sandy places than the latter. It is 

 not rare on the shores of Buzzards Bay at Woods Holl, Mass., although 

 it has been previously recorded only from Stony Creek, Conn. 



2) Veeeill, op. cit. This species occurs in abundance under stones 

 well above low water mark, being most common among dead barnacles. 

 The animals live many months in captivity, often breaking up into 

 small fragments, each of which regenerates the missing parts and bears 

 a close resemblance to a young individual. Perfect individuals may 

 be found which are not more than one-fiftieth the size of the original 

 specimes. It is my opinion that this is normally one of the common 

 modes of reproduction in this species, and that it takes place especially 

 after the deposition of the genital products. 



3) Veerill, op. cit. 



