[3] DEVELOPMENT OF THE OYSTER. 893 



Loven'' (1848) cauuot be passed by in silence, for liis exljaustive re- 

 searches on the development of the ModioJaria, Cardium, Jlontaeufa, 

 and MyUluH serve as the basis of our knowledge of the embryology of 

 the Laaiellibranchiata. xVlthough he has not observed the development 

 of the oyster, nor that of any other form of the groui) Monomyaria, he has 

 studied the representatives of widely different tamilies of the Dhnyaria, 

 and the results of his researches have, therefore, a wide scope. He de- 

 scribes in succession the expulsion of the polar globule, the cleavage, 

 the inclusion of the vegetative by the animal pole, the formation of the 

 velum, the origin of the shell, the origii/ and differentiation of the in- 

 testinal canal, &c. 



Davaine ' (1852) is the first who gave a detailed description of the 

 development of the oyster. According to him, the period during which 

 the oyster contains spawn extends from the beginning of June till the 

 end of September. He also observed spawn in the beginning of May; 

 but this must be attributed to the higher temperature of the shallow 

 basins where he observed it. After having directed attention to the 

 difficulties encountered in studying the development of the oyster, 

 caused by the circumstance that the eggs are retained in the cavity of 

 the mantle, whence they cannot be removed without bringing' their 

 development to a standstill and causing their death, he describes the 

 changes observed in the egg before it begins to segment. Sometimes 

 the germinative vesicle is missing altogether, while in others another 

 smaller vesicle is found attached to the germinative vesicle. Sometimes 

 there is a germinative vesicle half the usual size, while at other times 

 there are two of the same size, but only half the size of the ordinary 

 vesicle. (This description leads us to believe that Davaine had at that 

 period observed in the egg of the oyster some of the phenomena of 

 fecundation made known to us at the present day through the researches 

 of Hertwig, Selenka, Fol, and others.) 



The first stage of segmentation which was observed consisted in the 

 formation of four hiastomeres. These are of different sizes, and gener- 

 ally three small segments are found adherent to a larger one. These 

 segments, or cells, divide several times, diminish in volume, but increase 

 in number, so that finally the egg contains nothing but small cells. 

 Finally, the egg which had been round becomes heart-shaped. The 

 incision which gives this last-mentioned form to the egg soon disa[)pears, 

 and at two points of the outline vibratile cilia begin to appear. Oppo- 

 site these groups of vibratile cilia there appeared on the edge of the 

 egg a transparent streak. This is the beginning of the hinge of the 

 valves. During the iollowing period the vibratile cilia become longer 

 and more distinct, and a crown of them is formed, which indicates the 



("'S. Lov6n : Bidrag till Edniiedomtn om utvecMingen af MoUnsca Acepliala LamelU- 

 hranchiata. K. Vetensk. Alad. HandUjr., 1848. 



^C. Davaiue: Eccherches sitr la gen4raiion des hiiUres, with 2 pi., iu Comptts Bendus et 

 Mom. de '« Soc. de Biologic, vol. iv, 185'^, p. 21)7. 



