ANATOMY AND DEVELOPMENT OF HYDRA. 11 



which it is included. Through this the soft, protoplasmic 

 substance of the egg is gradually forced out, and finally the 

 whole egg has been expelled. 



The naked egg still holding on by a narrow point to 

 the remains of its former covering is now fecundated, the 

 testes discharge their contents, which become distributed 

 through the surrounding water, and the spermatozoa come in 

 contact with the egg and fix themselves by the head to its 

 surface, but they were never observed to penetrate its 

 substance. 



After fecundation the process of segmentation sets in. This 

 is preceded by the occurrence of two or three delicate pseudo- 

 podia-like processes on that spot of the egg which is diametri- 

 cally opposite to the point by which it still adheres to the 

 body of the Hydra, and, at the same time, a shallow depression 

 shows itself betAveen them. This depression deepens into a 

 narrow groove, which gradually extends through the substance 

 of the egg down to its base, and thus the division of the egg 

 into two germ-cells is completed. During this process the 

 bottom of the groove is seen to be set Avith pseudopodia, 

 Avhich present their characteristic movements of protrusion 

 and retraction. 



The entire process from the first appearance of the gi'oove 

 to the complete division lasts from two to two and a half 

 hours. The protoplasm of the two germ-cells shoAVS very lively 

 movements, and the second cleavage uoav begins. This com- 

 mences from the opposed surfaces of the tAVO first-formed 

 cells : it lasts from three to three and a half hours, and the 

 egg thus becomes divided into four germ-cells ; during the 

 process the formation of pseudopodia is still more marked 

 than during the first cleavage ; it occurs chiefly on the sur- 

 faces of division. The cleavage thus continues in the usual 

 binary order, and after the fourth cleavage is completed the 

 egg has assumed the " mulberry form.'' It subsequently 

 becomes quite smooth — the result not of the smallness of the 

 cells composing it, but of the filling up of the interspaces 

 betAveen the cell boundaries. 



After the completion of the segmentation tAvo forms of 

 germ-cells may be distinguished : one of these, consisting of 

 prismatic cells, constitutes a single layer, forming the surface 

 of the germ ; the other set consists of polygonally flattened 

 cells, and forms its inner main mass. These cells are all 

 naked protoplasm masses, and at first shoAV no trace of 

 nuclei ; a nucleus is subsequently formed in each, and this 

 arises independently of any pre-existing form element. 



A long discussion here ensues as to the essence of cell- 



