OF MASSACHUSETTS. 29 



Previous to the first cleavage the egg has taken on a pear-shaped appear- 

 ance, due to the formation of the yolk lobe. During cleavage this lobe 

 in many of the eggs became so constricted that the dividing egg had a 

 three-celled appearance. Then it gradually disappeared, in one case in 

 the course of seven minutes, leaving only a large and small cell. The 

 form of the different eggs during cleavage varies greatly, some dividing 

 with scarcely the appearance of a yolk lobe, others with prominent con- 

 strictions. 



The next cleavage (Fig. 8) divides the egg into four cells in a vertical 

 direction, and passes through the animal pole nearly at right angles to 

 the first cleavage plane, and a little to one side of the center. This 

 division forms three small cells and one large, the latter holding the 

 nutritive or yolk part of the egg, originally contained in the region of 

 the yolk lobe. The second cleavage occurs from fifty-five to eighty-one 

 minutes after fertilization, the average time being sixty-seven minutes. 



The third division (Fig. 9) is in a horizontal plane, dividing the four 

 cells into eight. The four upper cells, which lie next to the polar bodies, 

 are much smaller than the lower ones, and from this time forth are 

 designated as the micromeres, while the large lower cells are known as 

 the macromeres. During the process of cleavage the upper layer of cells 

 twists 45°, so that they alternate with the lower cells, furnishing an 

 excellent illustration of the spiral cleavage so common in nature. The 

 time of arrival at the eight-celled stage varies from fifty-eight to one 

 hundred and ten minutes after fertilization, the average being about 

 eighty-one. 



From this time on the micromeres divide rapidly into smaller and 

 smaller cells, during which the egg passes successively through sixteen, 

 thirty-two, sixty-four, etc., celled stages, finally forming a layer around 

 the macromeres. The average time of the sixteen-celled stage (Fig. 10) 

 is about one hundred minutes after fertilization. Cell division continues 

 until the single primitive ovum has become a compact mass of small cells 

 surrounding four large ceUs, the macromeres, resulting in a type of the 

 epibolic gastrula, which later becomes a true invagination by the further 

 division of the macromeres. From a surface view the animal is merely 

 a rounded mass of cells, still bearing the two small polar bodies (Fig. 

 11.) Soon the inner layer of cells forms an infolded cavity, the arch- 

 enteron or primitive digestive tract, which opens to the exterior. The 

 micromeres now make up the ectodermal, the macromeres the endo- 

 dermal layer. 



By this time .the surface cells have developed cilia, and the animal 

 acquires the power of locomotion (Figs. 12 and 13). It is important 

 that the scallop become active at this period of its existence, as other- 

 wise it would perish. In the laboratory the majority of the eggs settle 

 to the bottom of the glass dishes until this stage is reached. Doubtless 

 in nature the egg, unless held in floating masses or kept in suspension 



