INTRODUCTION 7 



If the species were such that both male and female had an equal 

 number of chromosomes in their somatic cells (say six for example), the 

 additional male chromosome would be designated as a "^/-chromosome," 

 and all four spermatocytes of the second order would carry three chromo- 

 somes, two with an x-chromosome and two with a y-chromosome. Should 

 the egg now be fertilized by a spermatozoon with an a:-chromosome, a 

 female would develop from it; if by one with a ^-chromosome, a male. 

 In this example the statement is made that reduction occurs in the first 

 maturation division, but it should be noted that reduction in some cases 

 occurs in the second division. 



The Spermatozoa.— It has already been stated that the final division 

 of the male cells results in spermatids that develop into spermatozoa. 

 The process of spermatogenesis is a complicated one which need not be 

 discussed here; suffice it to say the mature spermatozoon, in many ani- 

 mals, is an elongated organism with an enlarged head containing the 

 nucleus, a short middle piece, and a long flexible tail. By means of the 

 lashing motion of the tail the sperm immersed in fluid moves rapidly 

 about. Spermatozoa are very minute, although there is a considerable 

 range in size, those of Amphioxus being but 0.02 mm. in length; those of 

 the orthopteran Gryllotalpa vulgaris, 0.5 mm.; whereas those of the 

 amphibian Discoglossus attain a length of 2 mm. Wilson states that it 

 would take 400,000 to 500,000 sea-urchin spermatozoa to equal in volume 

 the egg of the same species. As Hegner (1914) says, it is not surprising, 

 therefore, to find that the number of spermatozoa produced by a single 

 male may be hundreds of thousands of times as great as the number of 

 eggs developed in a female. 



The Egg. — The egg of the animal is relatively very much larger than 

 the sperm; in fact the winter egg of the aphid may be nearly as long as 

 the body of the insect itself. Among animals that lay many in rapid 

 succession, the eggs are naturally relatively smaller than among those 

 depositing but few eggs. The amount of yolk in the egg is an important 

 factor in determining the manner in which the egg divides to form the 

 embryo. 



TYPES OF CLEAVAGE IN ANIMAL EGGS 



After fertilization, which will be discussed farther on, the egg cell 

 divides rapidly in 2, 4, 8, 16, etc., cells which continuously grow smaller 

 with repeated divisions. Two primary types may be recognized: holo- 

 blastic eggs with total cleavage and meroblastic eggs with partial cleavage. 



Holoblastic eggs may have a very small amount of yolk which is 

 distributed uniformly among the approximately equal-sized blastomeres, 

 as in Cerebratulus; or the yolk may be more abundant but not sufficient 

 to prevent cleavage as in amphibians. In the latter case the cleavage 



