IX MOLLUSCA 321 



distilled water, in which it is soluble. The egg-capsule is then opened 

 by a prick with a needle, and the embryo, as it floats out, is taken up 

 in a pipette and put into 30 per cent spirit, which is after a time 

 exchanged for 50 per cent spirit, and so by degrees the embryo can 

 be immersed in absolute alcohol. This method is employed when it 

 is desired to cut sections of the embryos. 



For the study of cell-lineage, however, another method is pre- 

 ferred. The whole mass of jelly with its contained egg-capsules is 

 thrown into a mixture of equal proportions of Perenyi's fluid and 

 water. The jelly at first turns milky but gradually becomes clear. 

 Then the capsules are opened by needles and the embryos float out. 

 They are put into 15 per cent alcohol, and then into 30 per cent for 

 twenty-four hours, and then gradually brought through higher grades 

 of alcohol into absolute alcohol. The right moment to open the 

 capsules must be carefully observed; if that time be allowed to 

 pass the jelly turns milky again. 



A method of staining with silver nitrate was also employed by 

 Wierzejski, and when successful it caused the outlines of the cells to 

 be indicated by brown lines. A-75 per cent solution is used ; in this 

 the capsules are allowed to stay until they are brown, then they are 

 opened and the embryos washed with alcohol. Wierzejski mounts 

 his embryos in a mixture of balsam and clove oil, which remains 

 sufficiently fluid to allow the embryos to be rolled about by the 

 motion of the coverslip. The coverslip is supported by thin pieces 

 of paper, or by thin pieces of glass tube drawn out to the proper 

 degree of tenuity. By slight modiflcations of this method the eggs 

 of all Opisthobranchiata and Pulmonata can be dealt with. 



The development of a Gastropod in which the larval stage begins 

 at the time when shell and foot have been formed, bears the same 

 relation to that of Patella as the development of Nereis sustains to 

 that of Polygordius. The most important differences are the ex- 

 tremely early indications of the future asymmetry, and the reduction 

 of the prototroch. In Crepidula, which has been studied by Conklin 

 (1897), and in Fiona, which has been worked out by Casteel (1904), 

 for example, only the anterior primary trochoblasts (i.e. the de- 

 scendants of la^ and Ib^) develop cilia, the greater part of the ciliated 

 band being formed from " secondary trochoblasts," which include the 

 tip cell 2bii and descendants of 2bi2 and 2b^S and even (in Fiona) of 

 certain cells of the third quartette. 



If this description is followed it will be seen that the velum 

 consists principally of two anterior lobes ; the circle is completed 

 in Crepidula by a band of ciliated cells which runs across the anterior 

 hemisphere of the larva in front of the apical cells, in Fiona by an 

 ill-defined ciliated area extending over the posterior part of the 

 front hemisphere, and in Physa not at all. When, as in many 

 Prosobranchs, the state of affairs is as in Crepidula, the eyes and 

 tentacles are excluded from the velar area, and hence the homology 

 of the velum with the Annelidan prototroch has been seriously 



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