No. 3.] THE GASTRULATION OF AMPHIOXUS. 587 



especially when such great variation is present. Sections of 

 embryos preserved in corrosive acetic will not show the differen- 

 tiation of the dorsal wall upon which we have mainly relied, 

 and after repeated examination of series of sections preserved 

 by this fluid, we have been obliged to give up all hope of orient- 

 ing with certainty the early gastrula stages. In the later 

 gastrula stages the shape of the embryo is sufficiently char- 

 acteristic to determine its orientation, but, in the early stages, 

 while in many cases the shape of the embryo might seem to be 

 sufficiently characteristic to determine the dorsal and ventral 

 sides, yet such a criterion alone is very unsafe. We do not 

 hesitate to say that many of the sections figured by some of 

 the more recent authors are in all probability oriented wrongly. 



Historical Review. 



Kowalevski gave a very brief account of the process of gas- 

 trulation in Amphioxus. A radially symmetrical invagination 

 is described ; the gastrula grows longer, and the small blasto- 

 pore lies at the posterior end. Later the blastopore shifts some- 

 what to the dorsal side. More recent writers have inferred 

 from Kowalevski' s brief account that the gastrula axis corre- 

 sponds to the embryonic axis, and while such a conclusion is 

 probably true, yet Kowalevski' s description is so very brief 

 that we can only infer this to be his meaning. 



Hatschek claimed to have been able to distinguish a bilateral 

 symmetry at the time when the invagination is completed. 

 We have shown that even at an earlier stage the bilaterality is 

 present and is shown by the differentiation in the flat plate 

 that is subsequently turned in, Hatschek noticed that the 

 large yolk-bearing cells around the vegetative pole of the 

 blastula, that subsequently form the endodermal plate, occupy 

 only about one-third of the circumference of the blastula wall, 

 and hence are, at first, too small to fill the entire inner surface 

 of the cap-shaped gastrula. He supposes that increase in the 

 volume of the endodermal cells takes place during invagination, 

 and this increase, he suggests, is brought about by absorption 

 of the fluid of the blastocoel space ; yet his figures show more 



