594 E. W. MACBRIDE. 



collapsed^ so that it is impossible to make out anythiug of the 

 limits of the coelomic cavities. The greatest difficulty I have 

 found in dealing with Amphioxus larvae is to stain the con- 

 nective tissue; and I have, in fact, only found this possible with 

 specimens preserved in osmic acid. Of course insufficient or 

 careless preservation in this fluid is valueless^ and leads as 

 usual to maceration. For the stages up to the end of the 

 gastrulation, however^ before any real differentiation of tissues 

 has taken place^ almost any reagent gives fair results. 



I do not intend in this paper to refer^ except incidentally, to 

 those features in the development of Amphioxus which have 

 been satisfactorily worked out, and about which there is a 

 general consensus of opinion. I may, however, remind the 

 reader that the eggs are spawned in the evening about 7 

 o'clock; segmentation takes place rapidly, so that by 11 p.m. 

 the blastula is complete and invagination has commenced ; 

 two hours later the gastrula stage is attained, and by 5 a.m. 

 the mesoderm has appeared, and the embryo which has been 

 for some time actually rotating within the egg-shell is hatched. 

 On the morning of the next day about 8 a.m. the embryo has 

 acquired the definite form of the larva, with a pointed snout, a 

 swollen pharyngeal region, and a very attenuated body, and 

 the mouth and first gill-slit are formed. No further develop- 

 ment has been attained with embryos reared in aquaria, and 

 in their natural habitat the rate of development is after this 

 very slow, extending over several months. I shall divide my 

 account of the subject into the following parts : — a. The gas- 

 trulation. h. The formation of the mesoderm, c. The fate of 

 the coelomic cavities, d. The origin of the atrial or '*^epi- 

 pleural^^ folds. 



1. The Gastrulation. 



A specimen of the youngest embryos examined is repre- 

 sented in PI. 43, fig. 1. It has the form of a regular sphere 

 bounded by one layer of cells of approximately equal size, and 

 is in fact a perfectly typical blastula. Although Hatschek 

 figures a difference in size of the cells in the two poles of the 



