376 HERBERT P. JOHNSON. 



of length ; hence the difference in productivity is not so great as 

 it seems when we consider the really considerable disparity in 

 size. 



Examination of the table also shows how variable is the 

 number of the somites, both anterior and posterior, in which 

 neither ova nor spermatozoa develop. In general, more somites 

 in the anterior region are destitute of sperm-cells than in the 

 posterior, while the reverse obtains with the non-ovigerous 

 somites. In every specimen but one (No. 5) more segments at 

 both ends of the series are without ova than are destitute of sper- 

 matozoa ; and often twice to three times as many somites contain 

 sperm as eggs. A better balance prevails in specimen 5, which 

 is exceptional, inasmuch as a single ovum has developed as far 

 forward as the 9th somite, one in front of the most anterior 

 sperm-producing one. 



Many somites, even within what might be called the ovigerous 

 series, fail to produce ova. Often the gap is only a single 

 somite, in other places it is two, three, or more. The longest 

 intervals have been found between the first ovum and the suc- 

 ceeding one (four somites in No. 2, six in No. 5). It is not 

 improbable that a thorough study by the serial section method 

 would bring to light minute, undeveloped ova in these apparently 

 non-egg-bearing segments ; for close inspection of the specimen 

 mounted entire has frequently revealed small ova which were 

 overlooked at first examination. In serial sections such ova 

 have occasionally been found in somites packed with sperm-cells, 

 entirely surrounded by the latter, and each accompanied by a 

 colony of what I regard as nurse-cells (Fig. 4). 



'The rich golden hue of the mature ova is due to the presence 

 of abundant yolk grains of different sizes (up to 35 microns in 

 diameter) distributed throughout the entire ovum except a peri- 

 pheral layer of finely-granular protoplasm (Figs. 3 and 5). This, 

 however, does not appear to cover the entire ovum, being absent 

 on the side adjacent to the intestine ; but the arrangement sug- 

 gests strongly that which prevails in the Arthropod egg. 



The nucleus, at first very large for the size of the ovum, be- 

 comes smaller and smaller proportionally as the size of the ovum 

 increases. (Compare Figs. 3 and 4.) The position is generally 



