Miscellaneous. 435 



MISCELLANEOUS. 



On the Development of the Heteropoda. By M. H. Fol. 



Although the excellent memoir of Krohn has furnished us with 

 numerous and precise details as to the larval development and the 

 metamorphoses of the Hetoropod Mollusca, on the other hand Ave 

 only possess very scanty and unsatisfactory data as to the com- 

 mencement of their evolution, notwithstanding that the genus Firo- 

 loides ought certainly to be regarded as one of the most favourable 

 for the study of embryogeny. 



The segmentation takes place in accordance with the same laws as 

 in the Pteropoda, except that the first four spherules of segmentation 

 are perfectly equal among themselves, and enclose the same pro- 

 portions of nutritive vitellus or protolecith and of formative vitellus 

 or protoplasm. Here also the nuclei disappear before each segmen- 

 tation, and are replaced by molecular stars. My memoir on the 

 development of the Geryonicles furnished in 1873 the first known 

 example, in the animal kingdom, of this mode of segmentation. 



The segmentation being completed, the embryonic sketch presents 

 the form of a cellular sphere, furnished with a central cavity, and of 

 which the histological elements are larger and more filled with 

 protolecith on the one side, the nutritive side, than on the opposite 

 or formative side. This latter bears in its centre the two polar 

 corpuscles. The nutritive side of the blastosphere enters afterwards 

 into the other ; and the aperture of invagination, which is at first 

 very large, gradually narrows ; it is the primitive mouth. This 

 opening of invagination occupies at first the pole exactly opposite to 

 that at which the polar corpuscles are ; but this arrangement soon 

 begins to change gradually. In fact one of the halves, which we 

 may call the ventral half of the embryo, begins to grow much faster 

 than the opposite half, so that it affects more and more a bilateral 

 symmetrical form. The part of the ectoderm of the ventral surface 

 which abuts on the primitive mouth constitutes a protuberance 

 which will become the foot. Between this protuberance and the 

 polar corpuscles a depression of the external lamella is produced, 

 namely the preconchylian invagination. 



The velum appears as a zone of cilia which passes between the 

 preconchylian invagination and the polar corpuscles, and unites at 

 the upper margin of the mouth. The polar corpuscles which adhere 

 to the point of the external lamella which w r as opposite to the 

 primitive mouth (that is to say, at the formative pole), are found to 

 occupy nearly the centre of the velum at the time when the larva 

 begins to turn. This relative displacement is due to the more rapid 

 growth of the tissues of the ventral surface of the embryo. Now 

 this ectodermic tissue, which occupies the centre of the velum, is 

 precisely that which gives origin to the cerebroid ganglia, the ten- 

 tacles, and the eyes. The cells from which these nervous organs are 



