I 



Embryology o/Ophiocoma echinata, Agassiz. 457 



sixteen clays old and showed no sii^ns of metamorphosis, but 

 probably had reached their adult form and size. 



The eggs, which were of an orange-red colour, after being 

 fertilized, threw about themselves a tough prickly egg- 

 membrane, which rendered preservation difHcult until it was 

 burst and the larva had escaped. 



At first the points and ridges of this chitinous egg- 

 membrane were quite high and regular, but soon were worn 

 down and became as shown in tigure 1. Under the outer 

 egg-membrane is still another, very thin and closely applied 

 to the developing larva. 



Segmentation was regular, and a blastula was formed, 

 consisting of cells of equal size or nearly so, and with a very 

 small segmentation-cavity. The long cylindrical cells com- 

 posing its walls before dividing flatten down and become as 

 nearly spherical as conditions will admit, as has been 

 described by Korschelt for sea-urchins and as is shown for 

 0. echinata in figure 1, which is a camera sketch of a section 



Fig. 2. 



Fiff. 1 



of a blastula. The growing larva now bursts the chitinous 

 membrane which encloses it, crawls out, and swims about in 

 the water. It is somewhat elongated and swims in the 

 direction of the axis connecting the animal and vegetable 

 poles, the animal pole, which is slightly pointed, preceding. 



As it moves from place to place it is continually revolving 

 on the long axis. At the time of hatching the mesenchyme 

 formation has just begun. It takes place by the rapid pro- 

 liferation of cells at the vegetable pole, no evidence being 



