236 RECORD OF CURRENT RESEARCHES RELATING TO 



mesoderm cells and ten endoderm cells being disposed evenly half on 

 each side of a median plane. 



Second period. — After the three germ-layers are established, the 

 mesoderm undergoes further multiplication, small daughter-cells 

 being divided oflf from the large primary mesoderm cells, and the 

 endoderm is invaginated, forming the archenteron. A special 

 accumulation of ectoderm cells (Scheitelplatte) on either side of the 

 anterior end forms the rudiment of the cerebral ganglion. At the 

 same time the embryo begins to rotate in the egg, cilia being 

 developed, chiefly in the neighbourhood of "the future velum, but at 

 first showing no regular arrangement. 



Eabl thinks it probable that the blastopore, although diminishing 

 greatly in size, never actually closes up : its anterior end corresponds 

 exactly in position with the future mouth.* 



On the dorsal side of the velum, the embryo grows out into a sort 

 of hump, the head-vesicle (Kopfblase). On the ventral side a smaller 

 prominence is formed, the rudiment of the foot. The shell-gland 

 appears as a slight depression in the ectoderm, almost immediately 

 opposite the mouth. 



The endoderm cells take in nutriment from the surrounding 

 albumen, and soon undergo diiferentiation into small cylindrical cells 

 lining the permanent digestive cavity, and large cells (albumen-cells), 

 external to the latter. The radula-sheath appears as an outpushing 

 of the ventral wall of the foregut ; a somewhat similar process from 

 the midgut, passing backwards and downwards, and ending blindly 

 close under the ectoderm, is the rudiment of the hindgut. At the 

 place where it terminates the ectoderm is raised up into a slight anal 

 elevation. 



The shell- gland increases considerably, extending inwards until 

 it nearly reaches the wall of the alimentary canal. Soon, over it and 

 the surrounding ectodermal area a delicate cuticular pellicle appears — 

 the rudiment of the shell. 



Up to a comparatively late period, the mesoderm consists of two 

 symmetrical rows of cells. Most of these cells — the number of 

 which remains small — are of moderate and equal size, but one in about 

 the middle of each row is conspicuous by being several times larger 

 than the others. This cell undergoes remarkable changes : a cavity 

 appears in its protoplasm behind the nucleus, and the cell sends out 

 two long processes, containing a continuation of the cavity, one 

 towards the " Scheitelplatte," the other towards the foot. The body 

 thus formed is the jjrimitive kidney ; it is ciliated internally, and both 

 its ducts, the anterior of which ends in a funnel-like aperture, appear 

 to open into the body-cavity. 



The velum has by this time become thoroughly differentiated ; it 

 consists of large, vacuolated, ciliated cells. 



Third period. — The embryo has now begun to lose its bilateral 

 symmetry: during the period all the more important organs take 

 their origin. 



* See, on this and other points, Prof. Laukester's remarks in the ' Quart. Journ. 

 Micr. Sci.,' xx. (1880) p. 103. 



