EVIDENCES FROM EMBRYOLOGY 165 



"In birds, the yolk of an egg is really a single ovum, distended to 

 an enormous size by food material. The sperm cell is very much 

 smaller and can be seen well only with a high-power microscope. It is 

 something like a tadpole in shape, having a small cell body, containing 

 a little nucleus, and attached to this a long, whiplike process which 

 beats rapidly while the cell is alive, enabling it to seek out and unite 

 with the large passive egg in the act of fertilization. Enormous num- 

 bers of sperm cells are produced by the male, but only one takes part 

 in fertilization. After the first has penetrated the membrane of an 

 egg cell, a change takes place in the latter which prevents the entrance 

 of others. 



"The sperm activates certain formerly inert substances in the egg 

 and the new combination cell (the zygote) starts almost at once to 

 produce a new individual." 



OUTLESTE OF ANIMLA.L DEVELOPMENT^ 



D. S. JORDAN AND V. L. KELLOGG 



The embryonic development is from the beginning up to a certain 

 point practically alike, looked at in its larger aspect, for all the many- 

 celled animals. That is, there are certain principal or constant 

 characteristics of the l;eginning development which are present in the 

 development of all mary-celled animals. The first stage or phenome- 

 non of development is the simple fission of the germ cell into halves 

 (Fig. 35, h). These two daughter cells next divide so that there are 

 four cells {c) ; each of these divides, and this division is repeated until 

 a greater or lesser number (varying with the various species or groups 

 of animals) of cells is produced. These cells may not all be of the same 

 size, but in many cases they are, no structural differentiation whatever 

 being apparent among them. 



The phenomenon of repeated division of the germ cell is called 

 cleavage, and this cleavage is the first stage of development in the 

 case of all many-celled animals. The germ or embryo in some animals 

 consists now of a mass of few or many undilTerentiated primitive cells 

 lying together and usually forming a sphere (Fig. 35, e), or perhaps 

 separated and scattered through the food yolk of the egg. The next 

 stage of development is this: the cleavage cells arrange themselves so 

 as to form a usually hollow sphere or ball, the cells lying side by side to 



' From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copyright 

 1907). Used by special permission of the publishers, D. Appleton & Company. 



