Nereidae 183 



readily fertilized artificially and development proceeds. Usually 100% 

 of the eggs cleave and, barring accidents, nearly all develop into normal 

 embryos. 



CARE OF CLEAVING EGGS 



The cleaving eggs require no further attention except that the water 

 should be changed several times during the next 12 or 15 hours. In the 

 meantime the eggs cleave and acquire cilia. After 24 hours the embryos 

 may be separated from the jelly and transferred to a clean dish of sea- 

 water either by pouring or by using a wide-mouthed pipette. Care should 

 be exercised to get rid of all decaying organic matter as soon as possible, 

 and this must be accomplished within 36 hours after insemination or 

 before. Cleaving eggs that are allowed to develop without frequent 

 change of water usually develop abnormally or die. 



SCHEDULE OF DEVELOPMENT 



The embryo becomes an early gastrula 12 or 15 hours after fertiliza- 

 tion, with the four large macromeres constituting the principal part of 

 the endoderm. It is ciliated and rotates in the jelly. It is a late gastrula 

 after 24 to 30 hours, the rate of development depending upon the tem- 

 perature. Between 36 and 48 hours the larva is a trochophore, at first 

 spherical but later somewhat elongated. On the third day the first three 

 segments of the worm body are completed and no additional segments 

 are added for several days although the embryo increases in size. It is 

 possible to keep the embryos until other segments grow but to do so 

 requires special feeding methods. The Nereis larva is unusually hardy 

 and easily cared for. The trochophores and early segmented larvae are 

 active swimmers but as the ciliary mechanism becomes inadequate they 

 depend more and more upon wiggling and creeping. 



CARE OF LARVAE 



The egg of Nereis is large and well supplied with yolk and oil so that 

 the larvae require no feeding during the first five or even seven days of 

 development. They are easily cared for because after two days they 

 have a tendency to settle to the bottom on one side of the dish and may 

 be transferred to a clean dish of seawater with a wide-mouthed pipette. 

 This should be done once per day or more frequently in hot weather. 

 After five days they may be fed upon diatoms but if it is desired to keep 

 them for several weeks it is best to transfer them to a large cylindrical 

 balanced aquarium, containing a dense culture of developing diatoms 

 which adhere to its sides. E. E. Just reared Nereis megalops to maturity 

 in such a jar of diatoms. The original stock of diatoms came from the 

 Fisheries Laboratory at Beaufort, N. C. 



