II 6 HOLMES. [VOL. I. 



from the posterior macromere D, as has been found in so many 

 other cases among mollusks and annelids. 



Serpulorbis squamigerns is a common mollusk on the coast of 

 southern California. The shell early loses all traces of its origi- 

 nally spiral form, and becomes bent and twisted in a very irregu- 

 lar way. Many individuals are often found tangled together, 

 resembling a group of worm tubes, and forming masses of con- 

 siderable size. The eggs are deposited in elongated capsules 

 attached by one end a short distance within the mouth of the 

 shell. In addition to the eggs the capsules contain numerous 

 small cells, probably follicular, which doubtless serve to nour- 

 ish the developing embryos. A large number of the eggs in 

 each capsule fail to develop normally, and sooner or later break 

 up into masses of isolated blastomeres. The cleavage of such 

 eggs is irregular, sometimes from 



C the start, but often the irregularity 

 appears only after the egg has de- 

 veloped for some time in an ap- 

 parently normal manner. As this 

 departure from the typical path of 

 development occurs at different 

 stages in different eggs, it is not 

 always easy to distinguish the nor- 

 rnal from the abnormal process of 



FIG. i. Eight-cell stage, seen from the Cleavage. 



animal pole. The dexiotropic origin of f^g fi rst two deavagCS are total 



the first quartette of micromeres is indi- 



cated, and the spindles in the angles of and equal, giving rise to a four-cell 



sta S eof the sual molluscan type, 

 in which two cells meet in a cross 



furrow at the vegetal pole. The next cleavage, which results 

 in the formation of the first generation, or quartette, of micro- 

 meres, is dexiotropic. The micromeres are rather small, as 

 is the rule in molluscan eggs, in which, as in the present 

 case, there is a large amount of yolk. At the next cleavage 

 the second quartette of micromeres are given off from the 

 macromeres in a laeotropic direction. The spindles appear 

 at one angle of the macromeres, but before the next division 

 the nuclei wander through the cell so that the spindles next 



