ENTOZOA. 89 



and a Distoma ; many individuals under the two latter forms being 

 developed out of one impregnated ovum.* 



Professor Sieboldl has traced the metamorphoses of another species 

 of Trematoda up to a certain point, the rest being made intelligible 

 by the analogy of those of the snail. He found that certain water- 

 fowl were, at particular seasons, infested with a small kind of fluke- 

 worm, the Monostoma mutabile. Rudolphi and others had described 

 them in the alimentary canal ; Siebold likewise found them in the 

 air-cells of the abdomen. He discovered this species to be viviparous, 

 and observed the act of bringing forth, when the Monostoma was 

 placed in cold or luke-warm water. The ova and embryo are de- 

 veloped as in the t^nia. The first germ-cell appears in the midst of 

 a thick granular germ-yolk ; it multiplies, and its progeny, as they 

 become smaller and more numerous, break through or divide the 

 yolk, and finally consume it. The embryos escape from the vulva, 

 close to the penis, and swim briskly, while the egg-shells fall to the 

 bottom. Sometimes an gq^ was excluded, containing the embryo, 

 which soon escaped. It is l-9th of a line in length, of a long oval 

 shape, with a truncated head and a rounded tail-end. On the upper 

 part of the head are a pair of square dark pigment-spots, which re- 

 minded Siebold of the eyes of the cercaria. In the terms of the 

 " alternate generation " theory the gregariniform being is the grand- 

 nurse " gross-amme ; " the ciliated monadiform larva is the great- 

 grand-nurse. This is an exposition of the main facts in figurative 

 language, but is not an explanation of them. What we require are 

 the conditions of structure, which give or admit of the power of pro- 

 creation without the coitus or act of impregnation in the procreating 

 animal. They appear to be these ; and they are revealed by examin- 

 ing the structure of the entozoon in question in the several stages of 

 its genetic cycle, comparing them with each other and with the 

 changes operated in the ovum by the reception therein of the sper- 

 matic principle. 



In the development of the monadiform larva of the monostoma or 

 distoma, either the vitelline membrane is metamorphosed into the 

 ciliated skin of the larva, or this is formed by the metamorphosis and 

 coalescence of the peripheral layer of the germ-mass. But, admitting 

 ■4;he latter process, no other parts of that impregnated germ-mass are 

 changed excepting those which form the external contractile and 

 ciliated integument ; there is no mouth, no stomach or other internal 

 organs, and no members ; the only change which has taken place in 

 the impregnated germ-cells is rather a kind of change of the relative 

 position of the essential spermatic or clear nuclear matter, which has 



* XCIII. p. 52. t XCIV. p. 321. 



