392 PROCEEDINGS OF THE ACADEI^IY OF [April, 



be seen to be rapidly spreading over the posterior region. As this 

 is being accomphshed it also grows greatly in size, producing the enor- 

 mous posterior vesicle which in figs. 109 and 110 extends far behind 

 the internal organs of the body. The importance of such an organ 

 must be considerable and, taken in connection with the early decrease 

 in size of the head vesicle, strongly suggests that its functional value 

 is similar in kind to that usually ascribed to the anterior or head 

 vesicle of other larvse. 



In all older veligers figured the original polar axis has become com- 

 pletely bent upon itself, a rotation of 180 degrees having occurred. 

 With regard to the median plane of the future embryo, the first cleavage 

 plane is obliquely transverse to this plane. AVhen the mesoderm is 

 formed it is thrown over toward this median plane, and from the first 

 is approximately bilateral in position (figs. 24, 31, 34). The elements 

 of both entoblast and ectoblast, which in late stages of cleavages lie 

 on the median plane, appear to be derived from cells of the early 

 cleavages which occupied similar positions. Little rotation, if any, 

 is apparent other than a certain amount of irregularity found in all 

 portions of eggs with equal or nearly equal cleavage. 



Conklin describes for Crepidula an entire rotation of the ectoblastic 

 cap at the time when the anterior and lateral cells of the fourth quartet 

 arise. Heymons shows a similar rotation in Umbrella. Such a change 

 of axis in the germ layers does not occur in Fiona, nor is there necessity 

 for it. The large macromeres of Crepidula and Umbrella are here 

 represented by small cells, which do not modify the positions of the 

 germ la3^ers at the time of their origin nor necessitate supplementary 

 rearrangement. 



Abstract. 



Maturation begins at the time of laying. Two polar bodies are 

 given off, the first of which may or may not divide. The un- 

 segmented egg of Fiona is rich in yolk, the spherules being com- 

 paratively small. In shape the egg is round, but shghtty flattened 

 in the direction of its polar axis. One to three eggs are found in a 

 roomy egg capsule. 



The early cleavage is strictly spiral after the dextral sequence. The 

 first quartet of micromeres are much smaller than the macromeres, 

 but with succeeding divisions the cleavage becomes equal in character. 

 After the four macromeres are formed they give rise to successive 

 quartets of micromeres. The first three quartets contain all the ecto- 



