HABITS, ETC., OF OEREBKA'L'ULUS LAOTEUS. 179 



branch molluscs, and has been described elsewhere (49). 

 Finally, in those cells where the cytoplasm becomes fibrillar 

 without forming any connection at all outside the cell, as 

 in ciliated epithelium, the contractions are in the form of 

 rhythmic vibrations. 



Sometimes it happens that the pseudopodia from two cells 

 which have anastomosed, and have been contracting in 

 rhythmic pulsations, become attached later to the pilidium 

 wall. In such a case the nature of the contractions also 

 changes, and they cease to be rhythmic, and become in all 

 respects like ordinary muscle. 



Histology. 



Methods. — A pilidium larva is nearly as hostile to preser- 

 vation as the medusa to which it is often compared. After 

 a trial of many different preservatives the best was found to 

 be platinum chloride, used either alone or in equal parts 

 with acetic acid. Corrosive acetic can also be recommended, 

 but is exceptionally difficult to wash out of the tissues. 



For staining, Delafield's haematoxylin, followed by eosin or 

 Orange Gr, gives good differentiation. 



Blastula. — A segmentation cavity appears very early, and 

 increases until the blastula consists of but a single layer of 

 cells surrounding this cavity. These cells are elongated at 

 right angles to the surface of the blastula, and vary greatly 

 in size and shape, so that they project unequally into the 

 central cavity, making the inner surface very rough. 



The cells at the inferior pole are longer and narrower than 

 the superior ones, so that there is thus early a differentiation 

 into ectoderm and entoderm. The nuclei are spherical, 

 comparatively large and uniformly granular ; they are nearly 

 always excentric in position, being much nearer the external 

 end of the cell. The cytoplasm contains so much yolk 

 material as to be opaque at first, but clears rapidly and 

 becomes transparent enough to show the central cavity 

 before gastrulation (fig. 55). 



