338 H. H. NEWMAN 



C. TWINS IN NORMALLY FERTILIZED EGGS 



In a considerable number of the control cultures that were 

 run as a check upon the results of parthenogenetic and of hybrid 

 experiments, no particular pains were taken to prevent crowding, 

 and as a consequence some of the larvae encountered unfavorable 

 growth conditions that retarded their development. The exper- 

 imental practice in the case of these controFcultures was to pour 

 off from the surface of each dish the upper third of the sea-water, 

 which contained most of the normal active larvae; for, especially 

 in somewhat crowded cultures, the larvae that are vigorous 

 enough crowd to the surface presumably for oxygen. The larvae 

 thus poured off from the surface are provided with fresh sea- 

 water and are therefore enabled to recover from any temporary 

 inhibiting influences under which they may have been living. 

 The original dish was refilled with fresh sea-water and kept for 

 some time for purposes of comparison. It was found that a very 

 large proportion of the larvae left after pouring off the surface 

 were subnormal in various respects. On the bottom of the dish, 

 apparently unable to rise from their position, were numerous 

 examples of exactly the kind of abnormal larvae, including the 

 various types of twins, that were found in parthenogenetic 

 strains. It therefore seems to be a safe conclusion that these 

 larvae are in fact parthenogenetic. Apparently these eggs are 

 in a physiological state in which they are incapable of fertihza- 

 tion, but are capable of cleavage without membrane formation. 



It is among the larvae that rise to the surface and are put into 

 fresh sea-water that one finds occasional instances of more ad- 

 vanced types of twins or double monsters than are found either 

 in parthenogenetic or in hybrid strains. Several types of these 

 twins will now be described in detail. 



a. One very interesting type of elongated larva was found 

 (fig. 38) in which two well-developed archentera appeared, one 

 in the normal position, the other, somewhat smaller, at the op- 

 posite end. Both archentera had widely open blastopores, 

 both were curved toward the ventral side and were sending out 

 the endodermal evagination for a mouth. In addition both had 



