16 



HUMAN EMBEYOLOGY. 



Simultaneously with the division of the oocyte II into the second polar body 

 and the mature ovum, the first polar body may divide into two parts. When 

 that occurs four cells are present within the oolemma at the end of the matura- 

 tion, i.e. the relatively large mature ovum and the three polar bodies (Fig. 19). 



The details of the maturation of the human ovum are unknown, and the above 

 account is based upon the phenomena which occur in other mammals. In mammals 

 two polar bodies are invariably formed, but in many the first does not divide into 

 two parts simultaneously with the formation of the second. The significance of 

 the differences which occur is not at present fully understood. 



Each of the four descendants of the oocyte I contains half the typical number of 



chromosomes, and those in the mature ovum 

 soon become enclosed in a new -formed 

 nucleus, which is called the female promicleus. 

 When the process of maturation is com- 

 pleted, the mature ovum differs from a 

 typical animal cell, inasmuch as it probably 

 possesses no centrosome and its nucleus con- 

 tains the chromatic substance of only half 

 the typical number of chromosomes. 



The first maturation division always 

 occurs whilst the oocyte is still in the ovary 

 and before the spermatozoon has entered 

 it. The second division takes place in the 

 FIG. is. SCHEMA OF MATURATION OF OVUM AT upper or middle part of the uterine tube, and 

 END OF ANAPHASE OF SECOND DIVISION a i ways a ft er the spermatozoon has entered 



The chromosomes 01 oocyte 11 have separated , ^ , 



the oocyte. 



If the mature ovum does not meet with 

 a spermatozoon it passes through the genital 

 passages and is cast off and lost; or it 

 breaks down, whilst still in the genital pas- 

 sages, into a detritus which disappears ; but 

 if it meets and unites with a spermatozoon 



/-"//. '^K" \ a zygote is formed, from which a new indi- 



I I $ FA vidual may arise, and in that case the polar 



( ^S?' bodies persist until the zygote has undergone 



J ; one or two divisions; but sooner or later they 

 .// disappear, probably breaking down into frag- 

 Femaiepro- \\< ments which are absorbed by the cells of the 



nucleus in. " V / 



Second polar bud with chromosomes 

 Chromosomes of second polar bud ' 



Oolemma :-<$d3HB[^te 



First polar body _;'_. 



Achromatic /./.- 

 spindle , //~"^ 



Chromosome? 



which remain ir 



mature ovuir 



into equal parts which have passed to the 



opposite poles of the spindle. 



Second 

 Parts of first polar 



Parts of first polar 



body S, 



Oolemma /'/ 



fex 



mature ovum 



Spermatocytes. When the male germ 



FIG. 19.-SCHEMA OF MATURATION OF OVUM. cells reach the P eri d f r wth the ^ are 

 END OF TELOPHASE OF SECOND DIVISION where called spermatocytes of the first order, or 



the four descendants of oocyte I are the mature spermatocytes I, which Correspond, mor- 

 ovum, with half the original number of ^holnmpallv with OOCvtPS I (Tiff 10") 

 chromosomes, and three polar bodies. . pnOlOglCaiiy, Wit ocyufc 



The spermatocytes he in the walls of 



the tubules of the testes or male sex glands, where their descendants become 

 converted into spermatozoa. 



They differ from the oocytes I in three important respects : (1) they have no 

 protective membrane corresponding with the oolemma of the oocyte ; (2) they are 

 not enclosed in follicles ; (3) the spermatocytes are not surrounded by definite 

 encircling layers of cells similar to the cells of the stratum granulosum. 



As the spermatocytes lie in the walls of the tubules of the testes they are inter- 

 mingled with other cells, the supporting and nurse cells, amidst which they undergo 

 their maturation divisions, and their descendants become embedded in the nurse 

 cells, where they are converted into spermatozoa. To a certain extent, therefore, 

 the nurse cells may be looked upon as corresponding with the cells of the ovular 

 cumulus which surround the growing oocyte. 



After it has reached its full growth each spermatocyte I, like each oocyte I, can 

 produce only four descendants, and the descendants, as in the case of the oocyte I, 



