No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 375 



Planorbis can be made to lay eggs even in the winter months. 

 Some of the snails I kept in glass dishes deposited eggs in 

 the latter part of January, and the cleavage of these eggs was 

 perfectly normal. 



The unsegmented eggs of Planorbis are almost spherical in 

 form and of a bright yellow color. The yolk, which gives the 

 t.^^ its yellow color, is somewhat more dense at the vege- 

 tal pole, although the difference is not strongly marked. The 

 fresh &%,^, when seen through the microscope by transmitted 

 light, shows a somewhat darker lower pole — the future ento- 

 derm — shading gradually into a lighter upper hemisphere — the 

 future ectoderm. The opaque matter of the Q,^g consists of 

 two elements, small granules and larger globules of deutoplasm. 

 The spheres of deutoplasm are found scattered through all 

 portions of the q%^ with the exception of a very small, clear, 

 protoplasmic area at the animal pole. The polar bodies are 

 small and clear. The first one is the larger and of almost 

 spherical form, and is carried on the top of the second polar 

 globule. The polar bodies remain in connection with the ^g'g 

 until quite a late period of cleavage, when they drop off and 

 disappear. The nucleus is situated in the small protoplasmic 

 area at the animal pole. 



The First Cleavage. 



At the beginning of the first cleavage the clear protoplasmic 

 area at the upper pole of the q^% increases in size and elon- 

 gates as the chromosomes are separated. The asters form 

 dense radiating masses of fibers which are clearly visible in 

 the living Qg%. Their general appearance is very similar to 

 those of Physa, which are figured by Kostanecki ('96). The 

 cleavage furrow appears first at the animal pole and gradually 

 extends downward on either side, finally surrounding the Qg^. 

 The constriction is deeper at the animal pole, as is the rule 

 with yolk-laden eggs in which the spindle usually lies above 

 the center. After the separation of the parts of the ^gg is 

 complete, the daughter-cells become nearly spherical in form 

 and come in contact at only a small portion of their surface. 



