262 INTERNAL FACTORS IV. i 



Boveri has traversed some of these statements of Driesch's. The 

 number of primary mesenchyme cells is, he maintains, a mean 

 between the paternal and maternal number, and the form of the 

 larva may be intermediate before the skeleton is developed. 

 Between these contradictory opinions it is not easy to decide, 

 but it may be mentioned that the number of mesenchyme cells is 

 peculiarly liable to variation, as Driesch has himself found. 



Before concluding this section we should mention some most 

 interesting experiments on heterogeneous cross-fertilization. 



Loeb was the first to show that by the addition of a small 

 quantity of calcium chloride and sodium hydrate to sea-water, it 

 was possible to fertilize the eggs of Strongylocentrotus purpu- 

 ratus with the sperm of Asterias ockracea. The matter has since 

 been more completely investigated by Godlewski, who has 

 succeeded in rearing Plutei from the eggs of sea-urchins fertilized 

 with the spermatozoa of the Crinoid Antedon. The necessary 

 condition of success is the prior treatment of both eggs and 

 sperm with an abnormally alkaline sea- water (about 1'75 c.c. 



m 



NaOH per 100 c.c. of sea-water). 



The sea-urchins employed were Sp/wer echinus, Strongylo* 

 centrotus, and Echinus. The eggs of the first reached the 

 gastrula stage, of the two last the Pluteus. Fertilization was 

 found to be perfectly normal, with production of a vitelline 

 membrane, rotation of the sperm-head, and so on. Cleavage 

 was of the Echinoid type (maternal) with formation of micro- 

 meres at the fourth division; no such micromeres occur in 

 Antedon, the fourth division being meridional in both hemi- 

 spheres. Primary* mesenchyme was formed (this again is absent 

 in Antedon], but the number of cells was slightly in excess of 

 the normal : and after gastrulation the skeleton of the Pluteus 

 was developed (except in Spit aer echinus) ; the larva of Antedon 

 of course has no skeleton. 



The Antedon chromosomes persisted and the nuclei of the hybrids 

 were intermediate in size between those of the parent forms. 



When an enucleate fragment of the egg of Echinus was fer- 

 tilized by Antedon all the processes mentioned above were normal, 

 except that segmentation was irregular, and that development 

 ceased after gastrulation. The death-rate of these larvae was high. 



