224 E. W. MACBRIDE. 



and " adult " planes respectively. A rudiment of the prseoral 

 lobe of the larva is retained, as we shall see, until the close 

 of the metamorphosis, and by means of it I found it possible 

 to determine the direction of the " larval " plane up till the 

 adult form has been almost attained. Hence, by cutting 

 sections parallel to the larval plane, one can follow the internal 

 changes of the metamorphosis step by step ; then when the 

 metamorphosis is complete it is possible to correlate with less 

 difficulty sections cut parallel to the two planes, and the 

 further history may be followed via, so to speak, the adult 

 plane. This was the course which I adopted ; and I also 

 penetrated back a considerable distance from the adult con- 

 dition into the stages of the metamorphosis by sections parallel 

 to the adult plane, and so confirmed results obtained by the 

 other method. For the youngest stages of all, which are 

 spherical, orientation is, of course, impossible, and one has to 

 trust to chance to getting sections in the proper direction ; 

 but it is fairly easy to recognise from their appearance when 

 this is so. 



General Account of the Development. 

 The ontogenetic history of the Asterina gibbosa may be 

 conveniently divided into three parts : first, the development of 

 the bilaterally symmetrical larva from the egg ; second, the 

 metamorphosis of this larva into the young star-fish ; and lastly^ 

 tlie gradual development of what we may term the young adult 

 into the sexually mature form. I have made no observations 

 on the segmentation of the egg, nor on the gastrulation; my 

 work, properly speaking, commences with the completed 

 gastrula, and my material was not suitable for observing the 

 development of the calcareous plates. On all these points I 

 intend, however, for the sake of completeness, to say a few 

 words, and my authority will be Ludwig, who, in his classic 

 research (12), has on these subjects left nothing to be desired in 

 point of view of completeness. I may add also that the figures 

 illustrating the changes in external form are copied from 

 Ludwin's memoir. The three figures illustrating the relations 



