1907.] NATURAL SCIENCES OF rillLADELPITIA. 351 



the number of cells differ in correspond iuju; stages of differentiation in 

 tliesc two forms, but e\cn in the same species there are variations and 

 irregularities in the later cleavages which are not present in the earlier 

 ones. When these irregularities of the later cleavages are compared 

 with the invariable features of the early ones, such as the segregation 

 of the ectoderm in three quartets of cells and the origin of the mesoderm 

 of the tnmk region from one cell of the fourth quartet, it is impossible 

 to avoid the conclusion that the early cleavages are of greater mor- 

 phogenetic value than the later ones. Furthermore, the conditions 

 found in Fulgiir, as compared with those in Crepidula, show that neither 

 the presence of a large amount of yolk in the egg before cleavage, nor 

 the modifications of the later stages, due to the presence of this yolk, 

 affect the localization of the morphogenetic materials and processes in 

 the earlier stages. The conclusion seems to be justified that the type 

 of this localization is a more general and fundamental character than 

 the form of gastrulation, or any other relatively late process in the 

 ontogeny. 



In Fulgur as in many other animals the egg shows polar differentia- 

 tion while it is still in the ovary, and even before yolk formation begins. 

 The yolk is laid down in all parts of the egg, and before matiu-ation one 

 pole is not noticeably richer in protoplasm than another. Witli the 

 maturation of the egg the segregation of the protoplasm and yolk 

 begins, most of the protoplasm passing to one pole and becoming the 

 future ectoderm, while the yolk remains at the opposite pole and with a 

 relatively small amount of protoplasm gives rise to the endoderm. 

 In the first tw^o cleavages this yolk is distributed equally to the four 

 quadrants of the egg, and since the blastopore forms at the vegetal pole, 

 the ectoderm must extend equally (but not synchronously) over the 

 yolk in all directions. In the early stages of this overgrowth the 

 anterior portion of the blastoderm extends more rapidly than the 

 posterior portion, and by this means the cerebral and buccal ganglia are 

 brought into close relation with the other orgaixs present in the posterior 

 margin of the blastoderm. 



In many animals the yolk is not distributed equally to the four 

 quadrants of the egg {e.g., Urosalpijix, Nassa, Denlalium, Chdtopterus, 

 etc.), and wherever this is the case a "yolk lobe" of considerable size 

 is present. When one macromere is much larger than the others a large 

 yolk lobe is attached to it ; when the cleavage is approximately equal the 

 yolk lobe, if present, is small. Crampton (1896) has shown that the 

 removal of the yolk lobe causes the absence of mesoderm in Illyonassa; 

 and in his beautiful "Studies on Germinal Localization" Wilson (1904) 



