380 OUTLINES OF CHORDATE DEVELOPMENT 



twenty to twenty-five minutes. In the mouse, at least, the 

 entire spermatozoon enters the ovum, but the tail soon disap- 

 pears. Some of the details of the process of fertilization in the 

 cat, are shown in Fig. 146. 



II 



Q 7*^yf<>f'l^7^ 



FIG. 146. Reconstruction of four sections through the fertilized ovum of the 

 cat. From Longley (combined from two figures). No zona pellucida is visible 

 in these sections. The corona radiata is disintegrating, s, Remains of second 

 polar spindle; /, first polar body; //, second polar body; d", sperm pronucleus; 

 9 , egg pronucleus. 



2. Cleavage 



Before taking up the details of cleavage and early develop- 

 ment, we should say, in preface, that in the placental Mammals 

 these processes of cleavage, formation of the blastoderm and 

 early cell layers, are in many respects unique and often very 

 difficult of comparison with the corresponding processes in other 

 forms. As mentioned above, many of these peculiarities result 

 from the fact that the mammalian ovumw r as originally markedly 



