OVUM. 



[105] 



tozoa into the ovum in others, suggest novel 

 and more general and extended views of the 

 process of fecundation, and while they add 

 certainty to the more limited observations of 

 the same kind made upon animals higher in 

 the scale, tend to prevent the adoption of 

 partial views in regard to these functions of 

 the animal economy. 



It is principally among the more highly or- 

 ganised Invertebrata that we meet with that 

 form of ovum in which the nutritive is com- 

 bined in considerable quantity with the forma- 

 tive yolk, and in which segmentation is partial, 

 such as the Cephalopoda, Insecta, Arachnida, 

 Myriapoda, Crustacea, and some of the Arti- 

 culate Worms. In by far the greater number 

 of the Mollusca, such as Gasteropoda and 

 Acephala, the ova belong to the smaller kind 

 with more or less complete segmentation, as 

 also in most of the Annelida, as Hirudinea 

 and Lumbricina, the Nematoid, Cestoid and 

 Trematode worms, with the Planariae, the 

 Rotifera, Echinodermata, Bryozoa, Acalephae 

 and Polypina. 



I now proceed to give a short statement of 

 the principal facts that have been ascertained 

 as to the structure of the ovum in these ani- 

 mals, and to state some details with regard to 

 some of those which are either best known 

 or which present phenomena of the greatest 

 interest. 



1 st. Large-yolked Ova with partial Cleavage. 

 Cephalopoda. The ova of this class of ani- 

 mals have already been referred to in connec- 

 tion with those of birds, scaly reptiles, and 

 cartilaginous fishes, to which they present in 

 some respects a greater analogy than to those 

 of almost any of the Invertebrata. The con- 

 siderable size of the germinal vesicle with its 

 multiple maculae, the large mass of the coloured 

 yolk (nutritive), composed of conglomerated 

 masses of yolk corpuscles, and the very limited 

 extent of the process of segmentation, which 

 affects only a round disc of the germinal part 

 of the egg, are all characters in which the ova 

 of the Cephalopoda, at least the Sepia and 

 Loligo, which have been fully examined, are 

 ascertained to be similar to those of the large- 

 yolked group. We owe the most of our 

 knowledge of the ova of this class and their 

 development to Kolliker's interesting treatise, 

 published in 1844.* The ova of the Sepia are 

 deposited singly, but are attached in numbers 

 close together by pedicles to the stalks of 

 Algse and other marine productions. Those 

 of Loligo arc arranged in sm?ll masses, in 

 which a number are enclosed in a general bag 

 or covering of gelatinous matter, which is at- 

 tached along with others of the same kind by 

 means of pedicles. I have found those of 

 Sepiola also thus enclosed in small pyriform 

 capsules. 



The ovum of Cephalopoda possesses a firm 

 laminated external covering or chorion, which 

 in some is darkened on the surface by the 

 colouring matter or ink, in others is trans- 



* Entwickelungs-gesch. der Cephalopoden, 4to. 

 Zurich, 1814. 



parent and' colourless. Immediately within 

 this outer membrane is situated a structureless 

 vitelline membrane, containing the mass of 

 yolk-substance, which is separated from the 

 membrane by a slight interval. It appears to 

 be ascertained that the chorion is formed by 

 superposition on the surface of the ovum dur- 

 ing its descent through the oviduct. 



In the ovary the ova are contained in slender 

 capsules, attached to the rest of the ovary 

 by narrow pedicles. When ripe the ova escape 

 from the capsules, in some species by an ir- 

 regular laceration, in others by a more regular 

 and defined opening, and, falling into the cavity 

 of the ovary, pass thence into the oviduct, 

 through which they are finally excluded. Fe- 

 cundation is believed to occur soon after the 

 escape of the ova from their ovicapsules or 

 in the earlier part of their descent through 

 the oviduct ; but this process has not, so far 

 as I am aware, been directly observed. 



The ova of the common Sepia officinalis 

 have an oval form, one end being much nar- 

 rower than the other. It is at this the pointed 

 extremity or narrow pole of the egg that the 

 germinal vesicle is situated, while the egg is in 

 the ovary, close under the vitelline membrane; 

 and it is at this part also that, at a subsequent 

 period, the process of segmentation and the 

 first formation of the embryo take place. The 

 narrow end is therefore the germinal pole. 

 This extremity of the egg is always turned to 

 the opposite side from the pedicle of the cap- 

 sule, which is attached to the middle of the 

 blunt or wider end. 



One of the most remarkable peculiarities in 

 these ova, is the extraordinary change which 

 the outer part of the yolk and the vitelline 

 membrane undergo during the greater part 

 of the time occupied by the growth of the 

 ovum in the ovary. This change, of which 

 the appearance had been known to some 

 previous observers, was first accurately- 

 described and explained by Kolliker. From 

 his observations it appears that at first 

 the ovarian ova are quite smooth on the 

 surface, and that at the time of complete ma- 

 turity of the ovum, or after its escape from 

 the ovary, the vitelline membrane and surface 

 of the yolk are also quite smooth ; but that 

 in the intervening time, that is, during the 

 greater part of the period of its growth, the 

 surface of the yolk is indented or marked with 

 peculiar grooves, into which folds of the vitel- 

 line membrane pass so as to line them to the 

 bottom, somewhat after the manner in which 

 the pia mater descends into the sulci of the brain, 

 but without the same convoluted form. This has 

 been represented by Kolliker in the Sepia, and 

 1 have observed it in this genus, and have con- 

 firmed in every particular that author's state- 

 ments as to this change. It appears that 

 at first these inflections of the yolk and 

 membrane begin as longitudinal folds, extend- 

 ing between the wide and narrow poles of the 

 ovum, and, gradually increasing, become at 

 last so deep as almost to meet each other in 

 the interior of the yolk. Subsequently they 

 are traversed by more numerous depressions. 



