52 



SYSTEMATIC ZOOLOGY 



shaped, and such a structure is known as a mesenchyme. 

 Finally this middle structure may consist of a definite layer or 



0u 



13 4 5678 



FIG. 34. Various forms of spermatozoa. 1, medusa; 2, nematode; 

 3, crustacean ; 4, ray or skate ; inder; 6, frog; 7, monkey : 



8, man, two views. (From Claus and Sedgwick's Text-book and 

 Parker and Haswell's Text-book. 1 



layers of cells and is then called a mesoderm. A cavity gen- 

 erally appears within the mesoderm, thus dividing it into two 



1 2 



Fir,. 35. 1, ovum of sea-urchin, showing the radially striated > j 

 membrane, the granular protoplasm within, anil in the center the 

 large nucleus containing the spherical nucleolus and granular net- 

 work. (Alter Hertwig.) 2, human ovum, showing the clear, radially 

 striated membrane, here called the zona pellucida, the finely granular 

 protoplasm within, and the coarser yolk or nutritive granules in the 

 midst of which the nucleus lies. 1 After Xagel.) 



layers, one adjoining the ectoderm, the other in contact with the 

 entoderm ; the cavity is known as the body-cavity or coelom 



(Fig. 53)- 



