xvn PEOTOCHOEDATA 587 



yolky eggs. But this idea is open to serious criticism. In all 

 alecithal eggs, even in those of Balanoglossus, the nucleus approaches 

 one pole of the egg when it undergoes division in order to form the 

 first polar body. 



The first polar body in the case of Amphioxus is formed after the 

 egg is shed from the ovary into the surrounding coelomic space, 

 which is termed the gonocoele. From the gonocoele the eggs escape 

 through two sht-like openings into the atrial cavity, along which they 

 pass back to escape by the atrial pore into the sea. In the sea they 

 are fertilized by the spermatozoa which are emitted by the male. 

 Both eggs and sperm are obtained simply by collecting specimens of 

 male and female Amphioxus and placing them in jars of clean sea- 

 water. They spawn usually in the evening about 6 p.m. The 

 spermatozoon enters the egg by the pole which is farthest from the 

 nucleus, and only after this happens is the second polar body given 

 off. After this the egg secretes a vitelhne membrane, inside which, 

 accordingly, the second polar body is enclosed, and this body remains 

 visible during the earlier part of the development and is used by 

 Cerfontaine as a landmark. The vitelhne membrane is formed as in 

 the egg of Echinus, by the coalescence of a row of spherical drops 

 emitted from the egg, the outer walls of which coalese to form a 

 coherent skin. The spermatozoon travels upwards through the egg 

 to meet the egg-nucleus, which descends to meet it. The compound 

 zygote-nucleus is, therefore, nearer the centre of the egg than was the 

 nucleus of the unfertilized egg, and it is surrounded on all sides hy 

 yolk-granules as in any other typically alecithal egg. 



The fertilized egg begins at once to segment, and it is easy to 

 keep the developing eggs in jars of clean sea-water, in which they 

 will live until the larvae hatch out about ten hours after fertiliza- 

 tion. These larvae live for a day or more until they have developed 

 a mouth and one gill-sht, further than this stage it has not been found 

 possible to rear them in captivity. Later stages are procured by 

 fishing in the water with a tow-net. The experiment, however, of 

 feeding these larvae on diatoms has never been tried : it is extremely 

 likely that an attempt to do this would be crowned with success. 



The eggs, in their earher stages of development, are best preserved 

 in the mixture of solution of corrosive sublimate and glacial acetic 

 acid ; but once the larvae have hatched, indeed once the gastrulation 

 is over, osmic acid is the best preservative. It is necessary to impreg- 

 nate them thoroughly with this reagent, in order to stiffen their delicate 

 ■ tissues and to prevent their collapsing in the process of dehydration. 

 Also it is absolutely necessary to embed first in celloidin, and then in 

 paraffin, if good results are to be obtained. The minute size of the 

 embryos renders it possible to obtain what is equivalent to a good whole 

 mount, by immersing the fragment of celloidin in cedar oil. The posi- 

 tion of the embryo in the celloidin block can, in this way, be accur- 

 ately ascertained, and so the difficult question of orientation is solved. 



The egg divides in the usual way into two ahd then four blasto- 



