220 SUMMARY OF CURRENT RESEARCHES RELATING TO 



several geographical races, as tlie forms mentioned by Baker, 

 Trembly, Eosel, Pallas, and Scbaflfer, differ a good deal in size, and in 

 the proportionate length of the arms. 



The young forms, just set free from their parent, have a re- 

 markable power of movement in the ectoderm ; this periodically 

 thickens into tubercles which are best developed in two circular 

 regions, but the number of tubercles is not constant. They may 

 gradually disappear, the hinder ones comj^letely, and the anterior 

 often give rise to a mammfeform papilla, which may become greatly 

 elongated and forked at the tip; a little later some of their cells 

 become converted into spermatozoa, canals being given off from the 

 central space protruded by the body-cavity, in which these elements 

 are developed. Still later, the hinder tubercles again become de- 

 veloped, either into buds (spring and summer), or ova (autumn). The 

 author is unable to explain why the male elements appear so much 

 earlier. 



It would seem that the buds of Hydra were not at first developed 

 in the interest of the species, but that they were merely blind sacs of 

 the body-cavity, which in time became provided with a mouth and 

 tentacles, and were rendered capable of leading a free existence. In 

 an examination of the causes of this phenomenon we have firstly to 

 note that when a Hydra is receiving more nourishment than it needs 

 it can only increase in extent by a system of folds, in other words, 

 diverticula are developed. Were these buds developed irregularly on 

 the anterior half of the body, the contractile power of the polyp 

 would no doubt be affected. But these considerations do not explain 

 why the buds get mouths ; the explanation of which may possibly be 

 that the body becoming too large, or the supply of food diminishing, 

 the parent animal would no longer be in a position to feed the buds, 

 which therefore, must develope the organs of independent nutrition, 

 and finally themselves break away and become independent of their 

 parents. And it is at any rate certain that, under experimental con- 

 ditions, the buds do break off earlier if the whole organism is subjected 

 to less favourable conditions of existence. 



A review and comparative account of the Hydroida leads to the 

 belief that in Hydra we have to do with a form which has been partly 

 degraded and certainly modified in adaptation to its fresh-water 

 habitat. 



Development of Renilla.* — The need of further studies on the 

 embryology of polyps in general is apjiarent to every zoologist ; and 

 Benilla, as a highly specialized form, presents a number of special 

 morphological problems, which can only be solved by a study of the 

 embryological history of the organism. Mr. E. B. Wilson therefore 

 selected the genus as a doubly desirable object for investigation, and 

 now publishes his results. 



The paper is divided into four parts. The first comprises an 

 account of the segmentation of the egg and formation of the germ- 

 layers ; the second a description of the formation of the tissues and 



* Pioc. Roy. Soc, xxxiv. (1882) pp. 384-8. 



