ON THE EXPERIMENTAL HYBRIDIZATION OF ECHINOIDS. 329 



The facts recorded in the foregoing paragraphs justify us therefore in suggesting 

 that the tempei-ature conditions of the sea- water, acting on the germ cells of our 

 Echinoids during their period of growth, may have played a part in changing the 

 type of inheritance in 1912. It is obviously useless, until we have more data at our 

 disposal, to discuss this question at greater length. 



15. Characters of the Hybrid Sea-urchins at the End of the First Year 



OF their Existence. 



In this section we are greatly indebted to Dr. Mortensen for the descriptions he has 

 furnished us of our larger hybrid sea-urchins. He has described, from the living 

 specimen, individuals from all our crosses that have survived the first year of their 

 existence. To render this account more intelligible, descriptions of the pure parent 

 species have also been included. These, with the exception of that of E. acutus, 

 have been based on small specimens raised from the egg in the laboratory, the 

 age of which was definitely known. The specimens described have been selected 

 so that all might be of as uniform size as was possible under the circumstances of the 

 case. It was impossible to be absolutely certain of many of the points in the 

 descriptions, as the examination of the hybrids had to be confined to observations 

 on the living animal, as we wished to keep as many of the hybrids alive as possible. 



It is remarkable that we have never been able to keep the pure-bred urchins ot 

 E. acutus alive for more than a few months after metamorphosis. This is, moreover, 

 singular, from the fact that the cross E. esculentus ^ X E. acutus $ grows very well 

 under laboratory conditions. We have had to rely, therefore, in our description of 

 E. acutus, on material obtained from the trawl, the exact age of which is unknown. 

 Small E. acutus are very seldom brought up by the trawl, and we have never been 

 successful in obtaining any at Plymouth smaller than 2 cm. across the test. If, 

 however, we are to judge from the rate of growth of E. esculentus ^ X E. acutus $ 

 hybrids, then these young E. acutus cannot be more than eight or nine months old. 

 It is natural to suppose that on the beds where the full-grown adult E. acutus are 

 found in such great numbers, it would be equally easy to obtain numbers of young 

 E. acutus, but all tlie young urchins brought up by the dredge or trawl are invariably 

 E. esculentus. It is possible that in the young stage, when tlie test is not more than 

 5 mm. in diameter, there may be no distinction between the young E. esculentus and 

 E. acutus. This is doubtful, on account of the difference between these two forms at 

 an age when they are not so very much bigger, which would seem to point to there 

 being a constant difference in pigmentation and size of the spines from the first. It 

 remains a fact, however, that the young E. acutus are remarkably difficult to obtain 

 from the beds where the adults are found in such great numbers. It is not infrequent 

 for 100 adult specimens of this form to be brought up by a single haul of the trawl, 

 and out of this number perhaps not more than six or seven will be under size. 



For this section we have furnished Dr. Mortensen with full data as to the age 



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