. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 28 OVUM. brought under the genus Tetrarhynchus. In fact, this kind of animal undergoes such re- markable changes in its transition from its first simple Echinococcus-like encysted form to its free segmented sexual Ttenia-like shape, that it is not wonderful that its history should have been obscure, and that great doubts should still prevail with someHelminthologists as to its origin, development, and zoological relations.* It has already been observed, that none of these three kinds of Cestoid Entozoa attain to sexual c
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 28 OVUM. brought under the genus Tetrarhynchus. In fact, this kind of animal undergoes such re- markable changes in its transition from its first simple Echinococcus-like encysted form to its free segmented sexual Ttenia-like shape, that it is not wonderful that its history should have been obscure, and that great doubts should still prevail with someHelminthologists as to its origin, development, and zoological relations.* It has already been observed, that none of these three kinds of Cestoid Entozoa attain to sexual completeness while they are en- cysted ; and it seems probable that they are all subject, more or less, to migration, in order to gain their free habitation in the alimentary canal of animals, where their segments ac- quire the male and female generative organs. The fecundated ova, produced in enormous numbers from each segment, do not in general, so far as is known, become developed into embryoes in the intestine of the animal in- habited by the Cestoid, but are evacuated along with the faeces, either separately after being discharged from the oviducts of the Cestoid, or before their discharge by the disjunction of the more ripe terminal segments from the rest of the animal. The migrations to which the ova and young of the Taenioid animals are thus made subject have hitherto opposed so great an obstacle to the observa- tion of their development, that we are as yet in possession of very few continued series of observations in which the whole progress of development from the ovum to the complete segmented animal has been traced. Some important contributions of this kind have, however, recently been made, and the great modifications which the views of comparative embryologists have undergone, from the novel and various aspects in which many of the phenomena of development are to be regarded in instances of alternate generations, have already indicated paths of inquiry by which th
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