HYBRIDS BETWEEN FUNDULUS AND MACKEREL 409 



nearly normal hybrids, those that reach an advanced stage of 

 development and occasionally hatch, show the least evidences of 

 paternal heredity — sometimes being pure maternal in their 

 chromatophores and in body form. If these larvae are absolutely 

 pure maternal, and I see no reason for doubting it at present, the 

 explanation of this hybrid phenomenon offers a very pretty prob- 

 lem in genetics. 



Loeb, as we have seen, classes all types of heterogenic Teleost 

 hybrids with his extreme interclass echinoderm 'crosses,' and con- 

 cludes that they are to be explained as products of a sort of 

 parthenogenesis, the foreign sperm playing merely a development- 

 initiating role and not functioning in later development and 

 heredity, except negatively as a retarding agent. 



The present studies show that this view is quite unacceptable 

 on three counts: 



1. The experiments just recounted prove that the sperm 

 functions in development and heredity. The same is no less true 

 for other crosses. 



2. These nearly normal pure maternal hybrids are merely part 

 of a graded series of types, the next grade showing a slight degree 

 of abnormality and a less positive pure maternal character, and 

 the next grade showing less normal development and positive 

 evidence of paternal heredity. Other grades show progressi\'ely 

 stronger paternal heredity and progressively less normal develop- 

 ment. The question would be pertinent, then: if the most suc- 

 cessful and apparently pure maternal hybrids are parthenogenetic, 

 what is the explanation of the rest of the series? There are no 

 sharp lines of demarkation between the normal and abnormal 

 nor between the pure maternal and the mixed type of hybrid. 

 If one results through the real cooperation of the sperm cell in 

 development, so do the others. 



3. The traditional factual basis for the belief in the partheno- 

 genetic nature of heterogenic hybrids is derived from the cyto- 

 logical studies of cross-fertilized eggs of echinoderms. In those 

 cases the usual situation in very wide crosses is one in which the 

 sperm head enters the egg but takes no part in cleavage, remain- 

 ing an inert lump in the egg cytoplasm. In Teleost hybrids the 



