EMBRYOLOGY 



spermatogonium divides into 

 "spermatocytesof the first order" ; 

 each of these divides into two sper 



2885 



maternal hereditary contributions, 

 (2) the restoring of the number 

 of chromosomes to the normal, 



Embryology. Spermato- 

 zoon of man. Above, 

 that of a horse 



matocytes of the 

 second order; 

 each of which 

 divides again 

 into two sper- 

 matids, which 

 become sperma- 

 tozoa. In this 

 case all the four 

 cells which are 

 descended from 

 a spermatocyte 

 of the first order 

 become sperma- 

 tozoa ; in the egg-cell lineage the 

 first polar body and the second 

 polar body are useless. The whole 

 subject is very difficult, but it is 

 very important, for maturation 

 probably affords opportunity for 

 new arrangements among the here- 

 ditary qualities which are borne, in 

 part at least, by the chromosomes. 

 It is held by many that it is in 

 the maturation-division that the 

 germ -cells are segregated into two 

 contingents differing hi the here- 

 ditary factors they carry, as is 

 suggested by the facts of Mendelian 

 inheritance (see Heredity). Ac- 

 cording to others, the segregation 

 of qualities is not confined to the 

 reduction -division. 



FERTILISATION. The intimate 

 union of the mature ovum and the 

 mature spermatozoon is called 

 fertilisation. The head of the 

 spermatozoon, penetrating the 

 ovum, swells up and becomes the 

 " male pronucleus " ; it moves to- 

 wards the reduced ovum -nucleus 

 the " female pronucleus"; the two 

 coalesce to form one nucleus the 

 " zygote nucleus " which will 

 presently divide. 



Fertilisation implies (1) the 

 mingling of the paternal and the 



(3) the introduc- 

 tion of a centro- 

 some (along with 

 the middle piece 

 of the spermato- 

 zoon) which 

 plays an import- 

 ant part in the 

 subsequent division of the 

 ovum, (4) a stimulus to the 

 ovum to divide or the re- 

 moval of some fetter that 

 was keeping the ovum from 

 dividing, and (5) a rapid 

 physical and chemical change in 

 the periphery of the egg-cell which 

 nips off the tail of the spermatozoon 

 and makes the egg-cell non-recep- 

 tive to other spermatozoa, the 

 entrance of which would cause 

 abnormal division. 

 By the brilliant 

 work of Loeb, 

 Delage, and SHELLS ; 



others it has been 

 shown that 

 " artificial p a r - 

 thenogenesis''can 

 be readily induced 

 in a large number 

 of ova (of starfish, 

 sea-urchin, of 

 some worms, mol- 

 luscs, fishes, and 

 of the frog) by a 

 variety of me- 

 chanical, physi- 

 cal, and chemical 

 stimuli. If the 

 eggs of the sea-urchin be placed for 

 a little while in sea - water whose com- 

 position has been slightly altered 

 by the addition of a small quantity 

 of magnesium chloride, they will 

 develop without fertilisation and 

 become larvae or even small sea- 

 urchins when restored to sea-water. 



EMBRYOLOGY 



If the eggs of the frog be pricked 

 with a fine needle, washed in blood, 

 and restored to fresh water, they 

 will develop without fertilisation 

 and become tadpoles or even frogs. 



It should also be noted that in 

 some cases the occurrence or non- 

 occurrence of fertilisation deter- 

 mines whether the ovum is to de- 

 velop into a female or into a male. 

 Thus the unfertilised eggs of a hive- 

 bee develop into drones. In other 

 cases, where there are two kinds 

 of spermatozoa, the nature of the 

 fertilisation settles the sex of the 

 offspring. Though the external 

 features of fertilisation in plants 

 are very different from those 

 typical of animals, the essentials 

 are the same. (See Sex.) 



SEGMENTATION. On the heels 

 of fertilisation comes segmentation 

 or cleavage, the egg-cell dividing 

 into many daughter-cells or blasto- 

 meres. The segmentation differs 

 according to the amount and dis- 

 tribution of the yolk. Thus it may 



BLASTODERM 



X TCHN ALLAYER 



YOLK BAG 



Embryology. Sectional view o! the embryo of a frog, illustrating how the 



brain, spinal cord, vertebrae, and alimentary canal are among the earliest 



parts to form 



ALBUMEN 



Embryology. Hen's egg shown in section 



be total and equal (as in the sea- 

 urchin) or total and unequal (as in 

 the frog) ; partial and discoidal (in 

 birds, reptiles, and most fishes) ; or 

 partial and peripheral (as in most 

 Arthropods). The result may be a 

 solid ball of cells (morula), or a 

 hollow ball of cells (blastula), or a 

 disk of cells (blastoderm). In each 

 division the chromosomes are split 

 longitudinally, and in some cases 

 it is possible for a time to demon- 

 strate that each nucleus has half 

 its chromosomes of paternal origin 

 and half of maternal origin. But 

 while the divisions bring about a 

 scrupulously equal partition of the 

 chromosome material (which may 

 perhaps carry the essential ger- 

 minal material of the race), there 

 may be dissimilar division of the 

 cell-substance of the ovum, so that 

 different kinds of building material 

 go to different cells. ." 



In the fertilised egg-cell of one 

 of the Ascidians or Sea-squirts 

 called Styela there are four or five 

 different kinds of substance which 

 occupy different positions, and 



