THE ANATOMY. REPRODUCTIVE SYSTEM 91 



protoplasmic mass referred to as surrounding the mature and nearly mature ova of 

 EudrilibS. 



The third mode of development of the ova in the Oligochaeta is especially 

 interesting, as it appears to show great resemblances to the development of the 

 spermatozoa in the same worms. It is characteristic of the Naids, the Enchytraeidae, 

 the genera Ilyodrilus and Phreodrilus among the Tubificidae. It has been followed 

 out by a good many observers; the eggs in these worms consist in their early 

 stages of masses of cells, which become detached from the ovary, and either find 

 their way into the egg-sacs or float freely in the body-cavity. Each mass consists 

 of a peripheral row of cells surrounding a central mass of protoplasm, -vPhich has 

 no nucleus and does not appear to be the equivalent of a cell. I have seen in 

 Phreodrilus the whole mass surrounded by a layer of flattened cells. Later one of 

 the cells increases in size and becomes a definite ovum ; when it breaks away the 

 other cells may in their turn become ova ; in this case the development is almost 

 exactly that of the spermatozoa, the only difference being that the peripheral row 

 of cells are not simultaneously converted into sexual elements. In the early stages 

 of the ovarian development of Ilyodrilus Stolc (3) represents the immature ova 

 as not separated by distinct boundary lines from the central non-nucleated mass. 



The spermatozoa and their development. — The spermatozoa in the Oligochaeta are 

 always filiform bodies and possess the power of free movement. As a general 

 rule there is no thickening at one end ; this was only observed by Vejdovskt in 

 the Lumbriculidae. The development of the spermatozoa has been studied by a 

 large number of naturalists ; the reader is referred to Bloomfield and Nasse 

 especially for details upon this subject. The principal facts appear to be the 

 following: the sperm undergoes its development in the sperm-sacs in those worins 

 (the majority) which possess these structures; the sperm mother-cells divide in 

 such a way as to form a peripheral layer of cells, not at first marked off from 

 a central, generally unnucleated mass of protoplasm ; this latter has been termed the 

 cytophore or the sperm- blastophore ; this structure is simply the remains of the 

 mother-cell, and possibly serves to nourish the growing spermatozoa; it is evidently 

 to be compared to the central mass of protoplasm round which the ova of 

 Phreodrilus, &c. are developed. 



§ a. Sperm-sacs. 



Nearly all the Oligochaeta possess sacs in which the sperm undergoes most of 

 the stages of its development, and which are on that account termed sperm-sacs; 

 this name is preferable, on the whole, to the earlier name of 'vesiculae seminales,' 



N a 



