[120] 



OVUM. 



or is rather a collection or aggregation of a 

 number of germs surrounded by a common 

 yolk ; in fact, as has been suggested, an ova- 

 rian sac containing a number of ova.* 



The manner in which the spermatozoa 

 reach the ova for fecundation does not appear 

 to have been ascertained with accuracy. 



Entozoa. The ovology of the Helmintha 

 or Entozoa has received considerable atten- 

 tion from physiologists, both on account of 

 the interesting nature of the phenomena pre- 

 sented by its study, and because of the anxiety 

 to discover the mode of production of these 

 parasites within the bodies of other animals. 

 From the researches on this subject which 

 have been prosecuted with great assiduity by 

 a number of observers in recent times, not 

 only have many doubtful points been solved 

 as to the origin of the Entozoa, and the views 

 of naturalists greatly modified in regard to 

 the history of these animals, but considerable 

 assistance has also been received in the elu- 

 cidation of general questions in ovology. I 

 will give a short sketch of what has been most 

 recently ascertained on this subject under the 

 three divisions of the Nematoidea, including 

 all the Round Worms, the Trematoda, and the 

 Cestoidea including the Cystica. All the 

 animals belonging to the first division are 

 bisexual, and the production of the embryo is 

 direct from the ovum, without metagenesis or 

 metamorphosis ; in the two other divisions 

 hermaphroditism prevails, and development is 

 indirect, or accompanied by metagenesis and 

 metamorphosis in the greater number. 



Nematoidea. The genital organs in the first 

 of these orders present the same favourable 

 circumstances as those of Insects for the ob- 

 servation of the structure and formation of 

 the reproductive elements in their successive 

 stages, as in the different parts of these tubu- 

 lar organs there are to be found at once the 

 spermatic cells and spermatozoa, and the ger- 

 minal cells and ova in every conceivable de- 

 gree of advancement from their earliest ap- 

 parition to the state of maturity. 



In the Ascarides and most of the round 

 worms, the upper closed extremities of the 

 two genital tubes of the female correspond 

 with an ovary, or rather as a portion of 

 it which may be regarded as a germ -form- 

 ing organ ; for in this upper part of the tube 

 are produced only the nuclei or nucleated 

 cells, from which the germinal vesicles derive 

 their origin. A second portion of the tube, 

 in which the granular yolk substance is 

 added, is to be looked upon also as a con- 

 stituent part of the ovary, and may be called 

 the yolk-forming or vitelligenous organ. Next 

 follows a constricted part of the tube, which 

 may be termed oviduct, in which the ova meet 

 with the spermatic corpuscles and undergo 

 fecundation. From this the ova pass into 

 the fourth compartment, a dilated portion 

 which has been called a uterus, and below this 



* Burnett's transl. of Siebold's Compar. Anat. 

 p. HO. 



the two genital tubes finally unite into a com- 

 mon vagina. 



In the Ascarides, the process of formation 



Fig. 85*. 



Development and fecundation of the ova of Ascaris 

 mystax. 



A. Earliest stage of the ova as they are found in 

 the ccecal or uppermost part of the ovarian tube ; 

 some from the highest part are mere molecules, 

 others a little farther down are minute nucleated 

 cells (germinal vesicles or germs of the ova), and 

 round these the primary yolk granules are be- 

 ginning to collect. 



B. Ova from the second part of the ovarian 

 tube in which they are closely pressed together 

 and arranged in a radiated manner round the axis 

 or centre of the tube. To the right, four of these 

 ova are represented adhering together ; to the left, 

 two ova are shown with their flat surfaces, and one 

 with its thin edge towards the observer. The ex- 

 ternal dotted line represents the surface of the 

 basement substance of the yolk in which the opaque 

 vitelline granules are deposited ; there is as yet no 

 vitelline membrane ; the germinal vesicle and 

 macula are very distinct. 



c. An ovum from the oviduct ; a faint marginal 

 line indicates the place where the vitelline mem- 

 brane is afterwards formed. The germinal vesicle 

 still visible, though obscured by the yolk granules ; 

 the ovum has now assumed an ovoid shape. 



D. Softened state of the ovum at a slightly later 

 stage, when it has met with the spermatic cor- 



