408 SUMMAEY OF CURRENT BESEARCHES RELATING TO 



2. Fecundation of the ova. — The mature ova, which measure 

 0*12 mm. in diameter, are protected by a very thick transparent 

 chorion, whose surface is covered with shallow indentations, and 

 between these a larger number of smaller dots, also pits, but, unlike 

 the former, in connection with fine lines or canals which extend 

 radially from the outer to the inner surface of the chorion. The 

 micropyle at the animal pole is a funnel-shaped projection, with a 

 large irregular opening, within which lay a number of highly re- 

 fractive globules. From the bottom of the micropyle two very large 

 polar globules arise, of which one, becoming much the larger, exhibits 

 a globular distal enlargement, in which an indistinct nucleus may be 

 detected. Four or five may be present, but only one has this globular 

 extremity. A layer of finely granular protoplasm was frequently 

 observed at the animal pole, where the polar globules arise. After 

 the detachment of the globules, a remnant is still seen, persisting as 

 late as the stage with eight segmentation-spheres. 



3. Segmentation. — The ovum divides after the general moUuscan 

 type, the earlier stages much resembling those of Planorhis. A 

 meridional division into two unequal parts, is followed by a successive, 

 not simultaneous, meridional division of the larger and smaller sphere, 

 at right angles to the former; a third division parallel to the equator, 

 a little nearer the animal than the vegetative pole, acts successively 

 on the four spheres, producing stages with five, six, seven, and eight 

 spheres. Beyond this stage the rhythm was not followed owing to 

 the abundant percentage of abnormal types. A blastosphere with a 

 slightly excentrie and oval cavity results. Four large, coarser cells 

 at the vegetative pole constitute the beginning of the endoderm. The 

 thick chorion falls away at an early stage in segmentation. 



4. Gastrulation. — The beginning of gastrulation is marked by the 

 increase in size and inward growth of the four primitive endoderm- 

 cells above-mentioned, of a wedge-like shape, at the end of segmenta- 

 tion ; they assume an oval form, and expand inwards into peculiar 

 club-shaped cells which nearly fill the segmentation-cavity. During 

 the inward growth, at first two, and then four or five cells at the 

 apical pole, becoming ciliated, form the beginning of the apical plate ; 

 and at the same time the velum is established by the appearance of 

 cilia upon each one of a double row of cells round the equator. The 

 radial symmetry of the embryo is soon disturbed by the appearance 

 of two large cells, one on each side of the four endoderm-cells, which 

 begin the transformation of the embryo into a bilateral organism. 

 These two " endo-mesoderm " cells divide, and one half becomes the 

 primitive mesoderm-cell, while the other, after remaining some time 

 in the mouth of the blastopore, is pushed in to become one of the 

 endoderm-cells lining the cavity of the mesenteron. The embryo 

 becomes somewhat lengthened through the elongation of the endoderm- 

 cells and the increase of the ectoderm-cells between the velum and 

 the mouth of the gastrula. The cells of the embryo-cap lose their 

 wedge-like form, and become somewhat flattened. 



5. Migration of the blastopore and appearance of tJie dorso-ventral 

 axis. — The cells filling the mouth of the blastopore divide per- 



