66 GENERAL EMBRYOLOGY 



The most obvious characteristic of the chromosomes is that 

 of numerical constancy. In different species of organisms the 

 number varies greatly but there is in general little if any 

 relation between the grade or relative complexity of the 

 organism and the number of chromosomes in its nuclei; closely 

 related species of a single genus may differ widely, e.g., Ascaris 

 megalocephala has four, A. lumbricoides forty-eight. In general 

 the number seems highest in some of the Protozoa. Where 

 there are very many minute chromosomes the difficulty of 

 counting them exactly is very great and it cannot then be 

 said precisely what or how constant the number is. In 

 Mastigella there are about forty, in Actinosphcerium one hundred 

 and thirty to one hundred and fifty, in Paramcecium about two 

 hundred. Among the Metazoa the smallest number is two, in a 

 variety of Ascaris, the largest known is one hundred and sixty- 

 eight, in Artemia. Frequent numbers are twelve, sixteen, and 

 twenty-four, but any number may be found within these known 

 limits. The number is practically always an even one in 

 somatic cells, even or odd in the germ cells or their immediate 

 predecessors. The numbers found in the tissue cells of some 

 of the familiar organisms are the following: rat, guinea-pig, 

 ox, sixteen; Amphioxus, salmon, salamander, frog, mouse, 

 man (female), twenty-four; earthworm, thirty-two; shark, 

 thirty-six; sea-urchin, eighteen in one species, thirty-six or 

 thirty-eight, in another; pine, onion, sixteen; lily, peony, 

 twenty-four. 



While the number of chromosomes is thus practically constant 

 it is not absolutely invariable and deviations from the normal 

 are now known to occur in several forms. Of course the most 

 frequent variation is the typical reduction of the number to 



-j, during certain phases in the for- 



mation of the germ cells (Van Beneden), or throughout the 

 gametophytic generation of many, perhaps most, plants. 

 But as we shall see later, this should hardly be called a variation 

 from normal. A deviation of an entirely different kind is seen 



o 



in a few cases where the chromosome number is = in cleavage 



