The molluscs, especially the gastropods and lamelli- 

 branchs, are considered easily available material for the 

 study of development processes; therefore they early 

 attracted the attention of embryologists . The division of 

 the ova of the gastropod molluscs is described in the 

 investigations of Van Beneden and Windischmann, and the 

 division of the ova of the lamellibranchs — in the work of 

 Loven. In the ova of Modiolaria marmorata {Mytilus aisoors) 

 Loven observed a maturation division and described it, as 

 a process of ejection of nucleolus (embryonic spot) . The 

 important result of his work was the establishment of the 

 fact that during the division of the ovum of Modiolaria, the 

 separation in the vegetative hemisphere of non-nucleated 

 lobes takes place which soon merges with one of the blasto- 

 meres. This phenomenon is repeated many times during the 

 time of the following divisions. Loven saw clearly the 

 process of division itself, i.e. the deviation of the ovum 

 into separate blastomeres, but he did not trace the changes 

 in the nuclei, the equal and mutual distribution of blasto- 

 meres . 



The separation of polar bodies in the ovum of the grey 

 slug is sufficiently clearly described in the above-mentioned 

 works of Van Beneden and Windischmann, although the division 

 following the separation of the polar bodies was inaccurately 

 described by them; they spoke about the formation of eleva- 

 tions on the surface of the yolk, divided by fissures. As 

 a result of this the entire surface of the ovum becomes at 

 the end as if embossed (BOSSELE) and resembles a raspberry, 

 The Belgian authors did not connect the formation of "the 

 yolk cells" with the appearance of the cuts on the surface 

 of the yolk; according to their concept, the yolk cells 

 originate under the superficial layer of the yolk. The 

 drawings illustrating their work show round bulges on the 

 surface of the developing ovum, and not its division as a 

 whole into separate blastomeres. The internal processes in 

 the divided ovum and blastomeres are neither described nor 

 illustrated in the drawings. Erroneous ideas of the processes 

 of division in the gastropodan molluscs were retained in the 

 work of 0. Schmidt appearing ten years later. 18 "The 



18. O. Schmidt, "Uber Entwickelung von Limax agvestis , " ARCH 

 ANAT., PHYSIOL. (1851), pp. 278 - 290 . 



533 



