GENERAL INTRODUCTION. xxiii 



The single cell from which a multicellular animal is developed is 

 known as an ovum. It may be derived from an epithelium or sub- 

 epithelium, ectodermic or endodermic (Coelenterata] ; or from a special 

 organ, the ovary, furnished with a duct and developed usually in the 

 mesoblast, sometimes from special cells set apart at a very early stage 

 of development, e. g. in some Insecta, perhaps in rare instances from the 

 endoderm (some Turbellaria). It may be naked, or provided with one 

 or more envelopes, derived from itself, from surrounding cells, or special 

 glands 1 . It may be hyaline, or it may be filled to a greater or less 

 extent with nutrient material, derived by its own vital energies from the 

 lymph-plasma of the body, from the products produced by the regressive 

 changes of surrounding cells (granulosa cells) of the ovary, rarely from 

 other cells. This nutrient reserve-material may be distinguished as food- 

 yolk or deutoplasm from the protoplasm with which it is mixed' 2 . Or 

 the nutrient material may be derived from a special gland, the vitellarium, 

 and be inclosed with the ovum in the egg-shell to be utilised as the ovum 

 segments (some Turbellaria^ Trematoda^ Cestodaf 1 - In one phase of the 

 life-history of the digenetic Trematoda, the Sporocyst or Redia, the re- 

 productive cell is one of a number of cells filling the central part of the 

 body and lining the body walls. These cells may perhaps be regarded 

 as collectively making up an undifferentiated ovary, i.e. as cells from 

 which, in another phase, the immature Fluke, the reproductive organs 

 are derived. 



As soon as the ovum has attained its definitive size, it very generally, 

 probably universally, gives origin to two polar bodies, or globules, or 

 directive vesicles. The ovular nucleus (germinal vesicle or vesicle of 

 Purkinje with nucleolus or germinal spot) approaches the surface, under- 

 goes karyokinetic changes, and finally one moiety is extruded with a very 

 small amount of protoplasm. After a brief period of rest the phenomenon 

 is repeated. The polar globules may themselves' divide again, and the 



1 An egg-shell must be carefully distinguished from structures inclosing a number of ova like 

 the cocoons of the Leech and Earthworm, which are secreted by the surface of the body. 



2 An ovary in which every ovarian cell becomes an egg, may be termed panoistic ; one in which 

 some only become eggs, others giving origin to secondary yolk ;or an egg-membrane, meroistic. The 

 terms are Brandt's, and were originally applied by him to Insectan ovaries. 



3 There can be no doubt that a vitellarium is essentially a part of an ovarium. Certain 

 Rhabdocoela prove this point remarkably well ; see von Graff, Monographic der Turbellarien, i. 

 Rhabdocoelida, Leipzig, 1882, p. 138, on the ' Keimdotterstock.' Granulosa cells, yolk cells, 

 epithelium cells connected with the egg, have very generally a similar origin to the egg itself. See 

 A. Thomson, 'Recent researches on Oogenesis,' Q. J. M. xxvi. 1886, p. 602, with lit. given p. 606, 

 and a paper by Korschelt, 'Uber die Entstehung, etc. der versch. Zellenelemente des Insekten- 

 ovariums,' Z. W. Z. xliii. 1886. 



