No. 3.] THE CELL-LINEAGE OF NEREIS. 377 



are wholly absent. The polar area slowly increases in size 

 until its diameter is about one-third that of the vitellus, and 

 meanwhile a very distinct star appears in its centre. Thirty- 

 five to forty minutes after fertilization the first polar cell is 

 extruded, and the second follows ten or twelve minutes later. 

 Viewed from the side at this period (Fig. 2) the vitellus appears 

 somewhat flattened on the upper hemisphere and is separated 

 from the membrane by a considerable space. I would call 

 attention to the fact that the polar cells differ slightly both in 

 size and in form, the first being pear-shaped or oval, while the 

 second is spherical and somewhat smaller. The corresponding 

 internal differences have not yet been investigated. 



II. General Sketch of the Development. 



The cleavage of the ovum takes place with a precision and 

 regularity which oft-repeated examination only renders more 

 striking and wonderful. Up to a stage when the foundations of 

 all the more important organs have been established (sixty-two 

 cells or more) the divisions take place with clock-like regularity, 

 the only perceptible variations being slight differences in the 

 time at which the individual blastomeres divide. Even these 

 differences are so slight as to escape any but the closest scru- 

 tiny. As development proceeds the variations become more 

 marked, and thus individual differences between the embryos 

 gradually become apparent. Yet these differences, as before, 

 are for the most part the result of slight time-variations in the 

 development of individual blastomeres and their progeny, and, 

 as far as can be determined, do not materially affect the end 

 result. The entire ontogeny gives the impression of a strictly 

 ordered and predetermined series of events, in which every cell- 

 division plays a definite role and has a fixed relation to all that 

 precedes and follows it. 



The events of the cleavage fall into three very marked periods 

 which I shall designate respectively as the (i) spiral, (2) tran- 

 sitional, and (3) bilateral periods. In the first period, which 

 extends to the thirty-eight-celled stage, the germ-layers are com- 

 pletely differentiated. At the same time most of the individual 

 blastomeres are differentiated into the parent-cells or proto- 

 blasts from which the future organs arise. The embryological 



