186 PROBLEMS OF FERTILIZATION 



attention. The failure of the spermatozoa to enter 

 other kinds of cells, even though there is abundant 

 opportunity for it, requires some explanation. In the 

 fertilization of Ascaris megalocephala, for instance, the 

 spermatozoa find their way up the long oviduct and 

 may be found in considerable numbers in the interstices 

 between the enormous swollen epithelial cells of the 

 oviduct, but they never enter the latter; in the lumen 

 of the oviduct the membraneless eggs, hardly larger than 

 the oviducal cells, are, however, all fertilized. The recip- 

 rocal of this relation is found in sperm agglutination 

 by secretions of ova but by no other tissue products. 

 The writer (1913) studied the latter question in detail 

 in the sea urchin and in Nereis and was unable to derive 

 even a trace of sperm-agglutinating substance, either 

 from the blood, which must contain secretions of all 

 tissues, or from any of the tissues individually; but 

 the ripe eggs always produced the agglutinating sub- 

 stance abundantly. There is thus a definite tissue 

 specificity in fertilization due to specific chemical 

 organization of the gametes, as one factor at least. 



II. SPECIES SPECIFICITY 



i. Hybrid fertilization. The extent to which species 

 specificity in fertilization is due to actual differences in 

 the chemical make-up of interacting substances has 

 caused much discussion. The problems united under 

 this head ally themselves with problems of tissue and 

 blood specificity and have the broadest biological bear- 

 ing. Specificity may concern different stages of ferti- 

 lization; in certain hybrid combinations the eggs do 

 not appear to react at all to the foreign sperm; in 



