1 88 ESSENTIALS OF BIOLOGY 



spermatozoon, is formed. The ovum, or spermatozoon, must 

 be regarded as an organism of the same kind as the parental 

 organism from which it was detached. It has, in a way, all the 

 characters of the parent, but those characters are potential in it. 

 In development (and still restricting the discussion to the ovum) 

 we see that these potentialities become realized so that, in the 

 developmental process, the ovum passes through changes such 

 that it gradually assumes all the specific characters of the 

 parent. 



Fertilization. In the majority of animal life-histories there 

 is conjugation of an ovum with a spermatozoon before embryogeny 

 begins, but this is not essential. The ovum, in many cases, can 

 be made to undergo embryogeny without being fertilized by a 

 spermatozoon and in the cases of parthenogenetic reproduction 

 there is no conjugation. We shall not, therefore, consider here 

 what is implied in the latter process. The changes of immediate 

 interest to us begin when the ovum becomes activated. 



69a. Types of Animal Life-histories. First we take those 

 which are usually called direct. There is embryogeny such that 

 the ovum develops within the body of the parent, as when the 

 development proceeds within the uterus of the mother. Or there 

 is embryogeny which is associated with an ovum that has much 

 yolk in connection with it — this is the case with the eggs of birds, 

 elasmobranch fishes, etc. The essential ovum, or '' germ," is 

 always a small body and as development proceeds it increases 

 enormously in mass. It must, therefore, be supplied with 

 nutritive material to allow of this increase. In many cases this 

 material comes from the maternal blood, or other body fluids, via 

 a placenta, or in some other manner. In other cases the nutritive 

 material comes from the food-yolk, albumen, etc., which is 

 associated with the ovum in the egg. Whenever, in such ways, 

 the ovum receives immediate and abundant nutriment embryo- 

 geny proceeds continuously and usually rather slowly and after 

 a period of " gestation " a young individual organism, having 

 recognizably the specific characters of the parent, is born or 

 hatched. Examples of such direct or continuous processes are 

 seen in the life-histories of man and other mammals, in birds, in 

 elasmobranch fishes, in squids and cuttle-fishes, etc. 



Indirect^ or discontinuous development occurs in the cases of ova 

 that are '' spawned " or emitted into the outer environment ; 



