OCR Text |
Show 1892.] AN ABNORMAL EARTHWORM. 1S5 of the somites. The anterior pair (fig. 1, ov.1) was borne upon the mesentery separating the 11th and 12th somites, and immediately dorsal to the coiled portion of the vas deferens which emerges from the latter segment. This pair is slightly the largest. The 2nd pair (ov.2) is situated on the 13th somite, and has all the typical relations of the ovaries as found in the normal worm; facing them, and piercing the mesentery 13/14, we find a small pair of oviducts (ovd.) with well developed receptacula ovorum (r.o.). Side by side with the latter structures, on the 14th somite, is a 3rd pair (ov.3) of these conical white bodies, slightly smaller than those in front, but resembling them in position and structure. On the 15th somite is situated the 4th pair (ov.1), the members being smaller, of unequal size, and no longer exhibiting the free tapering apex so characteristic of the anterior ones. The 5th, 6th, and 7th pairs of these bodies (ov.", ov.6, and ov.7) lie respectively on the 16th, 17th, and 18th somites, and show a marked decrease in size; they are relatively rounder and less regularly developed, the members of each pair being often unequal in size ; further, those on the 17th somite are slightly larger than the corresponding ones on the 16th. A rough examination with a hand-lens while the worm was still fresh showed that the free ends of these structures were composed of a number of large rounded cells, which at once suggested ova, especially as the pair of these bodies on the 13th somite possessed all the relations of the typical ovaries of the normal worm. The specimen was subsequently clarified in glycerine, and subjected to a microscopic examination, which entirely confirmed the last observation. Each of these bodies consisted of a number of rounded cells, smaller at the broad end, and becoming larger and rounder at the free end, the tapering apex being composed of a single row of these large cells (Plate XIII., ov^-ov.8, o.), in each of which can be seen a large round nucleus and one or two nucleoli. A comparison with the ovaries of a typical worm shows that the anterior pairs of these structures can in no way be distinguished from them, either in structure, position on their segments, or in shape. The four posterior pairs (fig. 2, ov.3-ov.7), however, rather resemble the ovaries of a very young or immature worm, being rounder and smaller, composed of smaller cells, and only showing one or two large round cells so characteristic of the adult ovary, while they are entirely destitute of that very characteristic free filamentous termination composed of ripe ova. From the consideration of the above description, together with a comparison of the figures given, there can be no doubt that each pair of these 7 cell-masses is the serial homologue of the single pair of ovaries of the normal Earthworm ; so that this interesting specimen, instead of possessing only a single pair situated on the 13th somite, is rich in the possession of seven pairs of ovaries, situated respectively on segments 12 to 18 inclusive. The condition and shape of the three anterior pairs suggests that they have already been functional in the discharge of ova into the body-cavity ; wdiile the four posterior pairs are in a more rudimentary |