ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 395 



base of the gonophores. Owing to the want of specimens of a certain 

 age Herr Wagner was unable to determine the layer from which the 

 genital cells were developed, but he does not doubt that they first appear 

 in the endoderm of the hydrorhiza, and that they are matured in the 

 ventral epithelium of the radial canals, whence they pass into the genital 

 sacs which form vertical folds along the radial canals. 



Digestion in Hydra.* — Miss M. Greenwood has made a careful study 

 of the process of digestion in Hydra. She finds that the ingestion of 

 solid matter is performed by slow advances over the prey of lip-like 

 projections of the substance of the polyp. During this action the 

 tentacles for the most part remain extended, having previously been in 

 contact with the prey or discharged their thread-cells into it. The 

 endodermal cells of the foot are more markedly vacuolated than those 

 of the body ; an ingested organism never, apparently, enters its cavity 

 but remains in that of the body, which it often distends greatly. The 

 digestion of inclosed food is efiected entirely outside the endoderm cells 

 which line the body-cavity of the Hydra, and among these cells two 

 types may be distinguished : (a) cells of pyriform shape destitute of 

 large vacuoles ; these hold many secretory spherules in hunger, and tend 

 to be emptied during digestive activity ; (b) ciliated vacuolated cells 

 which at times hold pigment. The water of the digestive secretion is 

 probably, at any rate in part, to be associated with the vacuoles of these 

 ciliated cells, for intracellular fluid is never so conspicuous as in the 

 fasting state, nor so little marked as after abundant nourishment. The 

 pigment, which is often conspicuous in the endoderm, is formed by 

 the activity of the cells, and is probably, as a rule, expelled into the 

 body-cavity during an act of digestion. The formation of secretion by 

 the endoderm and the loss of pigment are made inconspicuous during 

 the later stages of a digestive act, by the onset of absorption; this finds 

 expression in the gathering of proteid matter within the vacuolate 

 endoderm cells. This matter is deposited as a store of reserve sub- 

 stance in the basal part of the cells and eventually takes on the form of 

 spheres; Miss Greenwood believes that it is absorbed as fluid, forms 

 definite vacuoles bounded by the apical protoplasm of the cells, and is 

 by the indirect action of the protoplasm converted into the insoluble 

 form. 



It is probable that the excretory pigment takes its rise in some 

 residues of these proteid bodies, and it is possible that they may, at 

 times, be the source of fat. A large proportion of the spheres undergoes 

 final solution, and, when dissolving, they probably constitute the angular 

 particles of some authors. When this solution is effected it takes place 

 towards the apex of the inclosing cell ; the little masses of proteid are 

 moved upwards from their resting position, and fluid is secreted around 

 them by the investing layer of protoplasm. The medium in which the 

 digestive activity of a Hydra goes on is probabl\^ not acid. 



In a note a short account is given of the histological characters of 

 the endoderm of Hydra viridis. The " chloroplastids " of Lankester lie 

 especially thickly towards the base of the vacuolate endoderm cell, and 

 distally to them, in well-nourished specimens, are the true nutritive 

 spheres. Gland-cells do not form a conspicuous feature in the endoderm 

 of H. viridis; this may be because the presence of chlorophyll has 



* Jouru. of riiysioL, ix. (1888) pp. 317-44 (2 pis.)- 



