IV LUMBRICUS 



137 



The body of a Metazoon like the earthworm, composed of myriads 

 of cells all descended from one ancestral cell (the Zygote), is comparable 

 with a brood of Protozoa derived from a single ancestor. As in the case 

 of the Protozoon the process of syngamy when it takes place must as a 

 rule take place between cell-individuals not belonging to the same brood. 

 Ordinarily this is ensured by the gametes, the only cell -individuals 

 capable of conjugating, being specialized into two different kinds— micro- 

 and macrogametes — one of each being necessary for the process of 

 syngamy, and by the gametes of any one individual being all of the 

 one type. Occasionally however we find a particular species of animal 

 in -which the individuals are hermaphrodite — producing both male and 

 female gametes. In such cases the possibility of conjugation is commonly 

 guarded against in one of the following two ways. Either the male and 

 female gametes develop at different periods, so that at any one time 

 the individual is functionally of only one sex, or more or less complicated 

 topographical arrangements exist whereby the two sets of gametes are 

 kept rigidly apart. Lumbricus is hermaphrodite and it belongs to the 

 second of these two categories. 



The gonad consists, as is typically the case in animals which possess 

 a coelome, of localized thickenings of the coelomic epithelium. Of these 

 there are six, one pair of ovaries and two pairs of testes. The ovaries 

 (Fig. 67, ov) are situated on the posterior face of the septum between 

 somites XII and XIII, near its ventral edge and close to the median plane. 

 When fully developed they are small pear-shaped bodies, attached by 

 their base to the septum and tapering off into a fine filament which hangs 

 freely into the coelome of somite XIII. Microscopic examination shows 

 that this terminal filament is composed mainly of the eggs which have 

 reached their full size and are ready to drop off into the coelome. The 

 broader attached end is composed of small cells, which multiply actively 

 and which have not yet assumed the characteristics of the fully formed 

 gametes. The testes (Fig. 67, /) are very small lobed bodies which 

 project back into the coelome of somites X and XI from the septa 

 which bound these compartments in front. They are small and incon- 

 spicuous, partly owing to the coelomic space into which they project 

 being filled with a dense mass of gametes in all stages of development. 

 These various stages can be seen by examining a drop of the milky mass 

 with the microscope. The earliest stages as shed from the testis are 

 rounded cells. The nucleus of the cell divides and the cytoplasm becomes 

 constricted into two halves which however remain connected by a thick 

 bridge of cytoplasm. The processes of division of the nuclei and con- 

 striction of the cytoplasm are repeated over and over again, the end 



