PRODUCTION OF TWINS AND DOUBLE-MONSTERS 335 



it certain that true fertilization occurs in a large percentage of 

 the maturated eggs Moreover, it is easily possible to note that 

 all of those eggs that form a fertilization membrane and begin 

 cleavage continue their development to relatively advanced 

 stages. The principal difference between pure-bred and hybrid 

 Patiria eggs or larvae, is one of rate of development; for the 

 hybrid larvae from early cleavage stages on, are distinctly 

 retarded as compared with the pure-bred. Statistical counts 

 of two-, four-, eight-, sixteen-cell stages in pure-bred and in 

 hybrid stocks, fertilized from the same lot of eggs and at the 

 same time, showed a very striking difference in degree of advance- 

 ment. In one experiment, for example, when the pure-bred 

 stock showed a great majority of eight- and sixteen-cell stages, 

 the hybrid stock showed not a single egg with eight cells and a 

 majority of two-cell stages. A few hours later the difference 

 was even more striking. One cannot escape the conclusion, 

 therefore, that the developmental rate of the hybrid strains is 

 distinctly slower than that of the pure-bred strains, and that the 

 foreign sperm has exercised, in addition to its development- 

 initiating function, a development-retarding action. Whether 

 this effect is due to malcoordination or to some toxic influence 

 matters little for our discussion. We are at least certain that 

 there has been an early and more or less pronounced inhibition, 

 which is more or less completely recovered from in the later stages 

 of development. This inhibition is less profound, however, and 

 is more completely recovered from than is the case in partheno- 

 genetic eggs. Exclusive of twins adjudged to be the product of 

 parthenogenetic eggs, we find in hybrid Patiria cultures very 

 pretty examples of the more advanced types of larvae with 

 double, triple, and multiple archentera. Especially common 

 is the type in which a smaller secondary archenteron occurs at 

 or near the anterior end of the larva. A considerable number 

 of such double larvae have been seen to develop the ciliated 

 bands characteristic of the bipinnaria larva. A good series of 

 the more advanced type of double larvae in hybrid cultures is 

 shown in figures 29 to 32. It will be noted that in most cases 

 only the larger or primary archenteron undergoes any differen- 



