182 CHAS. B. WILSON. 



Its cells ai*e smaller, but the nuclei are about the same 

 size, and not being flattened the cells are held together 

 more securely. 



Both oral and aboral ectoderm are thickly covered with 

 fine cilia, which are longer and stouter at the apical plate 

 and the cilia row. 



The apical plate is an invaginated thickening of the 

 ectoderm. Metschnikoff regarded it "als eine Art indif- 

 ferentes Gehirn," and the fibres going from it as a nerve 

 commissure (36). 



Biitschli later maintained that the fibres must be regarded 

 as muscle, and that it was doubtful whether this plate really 

 was the central organ of the nervous system (11). 



Salensky considered Metschnikoff's assumption probable 

 from a morphological standpoint, since the apical plate can 

 be regarded as the homologue of that formed in the trocho- 

 phore larva. But he added that it was unlike this in its 

 simpler form, and it takes no part in the formation of the bi'ain, 

 and hence must be regarded as a rudimentary plate (45). He 

 then gives the first, and, as far as known, the last account of the 

 histological structure of the plate, closing with these words : 

 " The question as to the nature of the bundle (of fibres con- 

 nected with the plate) must be settled by a study of the 

 development history of the plate and the fibres." We have 

 just given the development history of the fibres, and have 

 found that they are undoubtedly muscle. The plate originates 

 in two or more cells found near the apical pole in a median 

 section of a blastula. These cells stain more deeply than the 

 others, and occasionally show karyokinetic figures (fig. 69) ; 

 otherwise they are like the remaining ectoderm cells. But 

 they divide and subdivide actively at right angles to the 

 surface of the blastula and form a group of cells, one layer 

 thick, a little anterior to the pole on the median line. 



The cells are cylindrical in form, and of the same diameter 

 throughout. Their nuclei are small at first, but become 

 relatively large as the cells diminish, and are always situated 

 close to the inner end of the cell. Their cytoplasm is more 



