4 Hybridization of Echinoids. 



characters of the abactinal system and the globuliferous pedicellariae, the most special- 

 ized Ecliinidae, such as Tripneustes, are apparently more advanced than any of the 

 Echinometridse, and it is therefore merely a matter of opinion whether Tripneustes or 

 Hderocentrotiis is considered the 'highest' of the regular Echini." 



These ideas, based upon a morphological and palaeontological con- 

 sideration of Echinoids, have an added significance and interest when 

 we find that Cidaris is also primitive in its development. Cidaris 

 shows some processes of development that have not been described, 

 so far as I know, for any other Echinoid, and in one phase of its early 

 development resembles the Crinoids and Holothurians more closely 

 than it does the Echinoids. The material has also given an oppor- 

 tunity for the study of the results following the insemination of the 

 egg of a species having a primitive type of development with sperms 

 of species having a modern type of development. 



Lytechinus and Tripneustes are about as widely removed from 

 Cidaris as it is possible for them to be and remain in the same class. 

 Lytechinus and Tripneustes are of the order Centrechinoida ; Czrfam 

 is of the order Cidaroida. Crosses between these forms are therefore 

 interordinal, and in themselves are of a good deal of interest. 



Conklin (1915, p. 176) has stated the generally accepted behef 

 regarding the respective potency of egg and spermatozoon in heredity: 



"At the time of fertilization the hereditary potencies of the two germ-cells are not 

 equal, all the early stages of development, including the polarity, symmetry, type of 

 cleavage, and the pattern, or relative positions and proportions of future organs, being 

 foreshadowed in the cytoplasm of the egg-cell, while only the differentiations of later 

 development are influenced by the sperm. In short, the egg cytoplasm fixes the 

 general type of development and the sperm and egg nuclei supply only the details." 



There has been a considerable amount of interest concerning the 

 time at which the influence of the sperm first becomes evident. 

 The material at hand enables us to push back our determination of 

 the time of appearance of paternal influence succeeding fertilization 

 to a point beyond that which has previously been reached and estab- 

 lished, but it is also convincing in its proof of the lack of plasticity 

 in the egg, of the inability of the egg to follow a system of develop- 

 ment which is not its own. The egg cytoplasm is the material which 

 is to be differentiated, but it does not seem to be able to harmonize its 

 inherent system of development with a foreign system. 



Most of the successful crosses between Echinoderms have been 

 within one suborder of the order Centrechinoida. The cleavage 

 pattern and the early development of the forms considered have 

 afforded no landmarks for the guidance of the observer. In this 

 Cidaris and Cidaris-Lytechinu^-Tripneustes material there is a defi- 

 nite, visible specificity of development, and it is possible to see the 

 exact period at which the disharmony between two systems of devel- 

 opment begins. 



