14 



HUMAN EMBEYOLOGY. 



Oolemma 



Nucleus 



FIG. 12. SCHEMA OF MATURATION OF OVUM, 

 EARLY PART OF PROPHASE OF FIRST DIVISION. 



Oolemma 



Nucleus 

 Chromosome 



//; \V\ 



from coming into close contact with the maternal tissues until it has attained the 



proper stage of development. 



The exact origin of the oolemnia is unknown. It must be formed either by the 



action of the cells of the stratum granulosum, or by the action of the oocyte, or by 



interaction between the two ; but, up to the 

 present, opinions regarding the origin are 

 divided. 



It is stated that processes of the cells of 

 theovular cumulus pass through the oolemma, 

 forming the radial striae, and become continu- 

 ous with or lie in close association with the 

 protoplasm of the oocyte ; and it is probable 

 either that the processes are used as pabulum 

 by the growing oocyte, or that they transmit 

 nutritive material to the oocyte. 



The Body of the Ovum. The cell body, 

 originally called the yolk, consists of spongio- 

 plasm and hyaloplasm. 



The Deutoplasm. The deutoplasm con- 

 sists of a number of more or less highly 

 refractile granules, of varying size, which are 

 embedded in the cytoplasm. They are largest 

 in size and are most closely aggregated to- 

 gether in the region around the nucleus, 

 where they form a definite deutoplasmic 

 zone. But in the human ovum and in the 

 ova of the majority of mammals, as contrasted 

 with the ova of birds, reptiles, and amphibia, 

 the amount of deutoplasm is relatively small ; 

 and for this reason the human ovum is 

 classified as oligolecithal, the term telolecithal 



FIG. 13. SCHEMA OK MATURATION OF OVUM IN being applied to ova in which the deutoplasm 



The chromatic is present in considerable amount, as in the 

 ova of the frog; whilst the ova of birds, 

 many reptiles, and the monotremes amongst 

 mammals, in which the deutoplasm greatly 

 preponderates over the cell protoplasm, are 

 termed eutelolecithal. 



The deutoplasmic granules are believed 

 to serve as a store of nutritive material 

 which is utilised during the early stage of 

 the growth of the zygote, during which 

 they disappear. (See note 2, p. 79.) 



The Nucleus. The nucleus, formerly 

 called the germinal vesicle, is a spherical 

 vesicle of comparatively large size ; its 

 diameter, which measures 50//,, being one- 

 fourth of the diameter of the oocyte. It 



FIG. 14. SCHEMA OF MATURATION OF OVUM AT usually lies excentrically in the cytoplasm. 



Its constituent parts are a nuclear mem- 

 brane, surrounding the karyoplasm, which 

 is separable, as in ordinary animal cells, into 



(1) an achromatic reticulum, the linin ; (2) chromatie substance, which is embedded 



in or closely connected with the strands of the linin ; and (3) the nuclear juice, 



which fills the meshes of the reticulum ; and it contains usually one, but sometimes 



several nucleoli. 



The Centrosome. The centrosome is not always very evident. It is usually 



present during the growth stage of the oocyte, and it disappears when the first 



maturation division commences. It may contain one or two centrioles, and it 



PROPHASE OF FIRST DIVISION. 

 thread has divided into twin chromosomes. 

 Each twin may be assumed to consist of a 

 maternal and a paternal part. 



Oolemma 



Achromatic spindl 

 Twin chromo- 

 somes / 



END OF PROPHASE OF FIRST DIVISION. The 

 twin chromosomes lie at the equator of the 

 achromatic spindle. 



