[124] 



OVUM. 



in most but not in all of which segmentation 

 is complete. This process was first made 

 known through the interesting researches of 

 Kdlliker*, in Muller's Archiv., 1843, p. 68, 

 and Bagge, in his Inaugural Dissertation.-j- 

 The memoir of Reichert in Muller's Archiv., 

 1847, contains very correct views as to the 

 formation of the spermatic cells. 



The accompanying figure from Meissnerjl, 

 gives a representation of a remarkable form 

 of the external capsule of the ova occurring in 

 some of the Gordiacei (Mermis nigrescensj. 



Fig. 90*. 



Mature ova of Mermis nigrescens. {From Meissner.) 



This figure is introduced to show the very pe- 

 culiar capsule in which the ovum is enclosed. 



a. Ovum taken from the uterus with embryo 

 enclosed ; the chorion and shell capsule with cha- 

 lazae or brush-like processes attached to the latter. 



b, c. The shell capsule c burst across the equa- 

 torial groove, allows the ovum b to escape with the 

 chorion and embryo contained within it. 



The ova of Trematoda are generally of a 

 long-oval form, and of middle size. They are 

 enveloped by a shell membrane of consider- 

 able firmness, and which is not unfrequently 

 of a dark brown colour. The yolk-sub- 

 stance contains fat corpuscles simple and 

 compound ; and there is a germinal vesicle 

 present, which, however, from the deep 

 colouration and other circumstances, is often 

 very difficult of detection. 



In these animals an interesting peculiarity 

 in the arrangement of the reproductive organs 

 exists, in the separation of the germ-forming 

 and yolk-forming portions from each other ; 

 in the first of these organs germinal vesicles 

 or clear nucleated cells alone being produced, 

 in the other the opaque granular fatty matter 

 which furnishes the vitellus. This arrange- 

 ment was first described by Von Siebold in 

 1836.$ The germ organ is generally in the 

 form of a rounded sac, which is filled with 

 the nucleated germ-cells or vesicles in various 



* See his admirable memoir on the first changes 

 in the fecundated ovum, principally referring to the 

 Entozoa. 



f Dissert, inaug. de Evolutione Strongyli auri- 

 cularis et Ascaridis acuminatae, Erlangae, 1841. 



t ZeiLsch. fur Wissen. Zool. vii. pi. ii. 



Wiegmann's Archiv., 1836, p. 217, Tafl. vi., 

 and Muller's Archiv. 1836, p. 232, Tafl. x., fig. 1. 



stages of development. The vitelline organ is 

 double, each one consisting of ccecal tubes, in 

 which the opaque granular yolk-substance is 

 secreted.* The ducts of these two organs 

 meet in a common cavity or uterus, and the 

 germs descending into this cavity are there 

 enveloped by a portion of the vitelline mass, 

 and shortly afterwards an enclosing vitelline 

 membrane is formed. These animals being 

 hermaphrodite, the vas deferens of the male 

 organ or testicle leads into the uterine cavity ; 

 and it would appear, therefore, that in many 

 cases, if not in all, impregnation takes place 

 by the access of the spermatic corpuscles to 

 the elements of the yolk and germinal vesicle, 

 just at the time when they are brought toge- 

 ther to form the ovum. 



This separation of the germ-forming and 

 yolk-forming parts of the ovarian organ, which 

 is so apparent in the Trematoda, is not in 

 truth so great a departure from the more 

 familiar structure of other animals as might at 

 first be thought ; for, as Leuckart has well 

 observed, there are other examples of the 

 same disposition, or an approach to it. Thus 

 in Insects and in Nematoid Worms, as we have 

 seen, it is from distinct parts of the genital 

 tube that the germs and yolk are produced ; 

 and more or less of the same arrangement 

 exists in all instances in which the form of 

 the ovary is tubular. 



The Cestoidea present a great similarity 

 to the Trematoda in the arrangement of the 

 organs by which the ovum is formed. Indeed, 

 notwithstanding the difference of their antece- 

 dent stages of development, the structure of 

 the mature sexual joint or proglottis of the 

 tapeworm, offers so great a resemblance to that 

 of some of the Trematoda, that they have been 

 regarded as identical by several recent obser- 

 vers. In each sexual joint of the tapeworm, 

 the testicle and the two parts of the ovarian 

 organ coexist, and, as stated in an earlier 

 part of this article, arrive at maturity simul- 

 taneously in the posterior or oldest segments 

 of the body. Van Beneden has, in his recent 

 work on the Cestoid Wormsf , described very 

 clearly the structure and relations of the ger- 

 migenous and vitelligenous parts of the repro- 

 ductive organs in the complete segments or 

 proglottides of a variety of Cestoid worms. 

 The ova originate in the first mentioned of 

 these organs as germinal vesicles, and, passing 

 into the vitelligenous part, meet with the 

 yolk-masses formed there. Near the same 

 place is situated the seminal vesicle, from 

 which, doubtless, the spermatic substance 

 easily reaches the ovum as it descends in the 

 course of its formation. The ova then ac- 

 quire an external envelope, and pass into the 

 cavity termed a uterus. As they corne to be 

 accumulated in gradually increasing quantity 

 in the latter cavity, they distend it to a great 

 degree, so as to cause it to pervade in various 

 forms, ramified and others, the whole body of 



* See also Thaer on this subject, in Muller's 

 Archiv., 1850, p. 626. 



f Mem. sur les Vers Cestoides. Acad. Roy. de 

 Belgique, torn. xxv. 1850, see plate B. 



