1908] Davis. Life-history of Doliclwglossus. 213 



with here and there light spots indicating the position of cells 

 from which secretion has been discharged. 



OCCURRENCE OF HALF EMBRYOS AND DOUBLE EMBRYOS. 



The blastomeres of the early cleavages are loosely held to- 

 gether. This fact may account for the appearance of various 

 small embryos. No careful study was made of them; indeed a 

 great many were destroyed or not preserved. From the few 

 notes I made the following may be of interest, particularly in 

 view of the recent studies in the development of egg fragments. 



The occurrence of what may be called half embryos was noted 

 in several stages. The earliest was where two blastomeres of the 

 four-celled stage had been destroyed. The next stage observed 

 consisted of eight blastomeres. This stage corresponds to the 

 third cleavage except in the smaller size of the blastomeres. 

 Another stage was an early blastula about half normal size. 

 Within the egg capsule were the disintegrating remains of a bias- 

 torn ere, probably the companion of the one that had developed. 

 The latest stage observed was one where the ciliary band had 

 appeared. It was small but otherwise normal. In the egg capsule 

 was a disintegrating blastomere. 



The occurrence of double embryos was observed in a few 

 stages, but none later than the blastula. The embryos seemed 

 normal except in size. 



These observations would indicate that isolated blastomeres 

 in D. pusillus may develop into embryos, as they are known to be 

 able to do in Amphioxus and some other animals. 



COMPARISON OF D. PUSILLUS WITH AMPHIOXUS WITH REFERENCE TO 

 ORIGIN OF BODY CAVITIES. 



Since Bateson called attention to certain points of similarity 

 between Balanoglossus and Amphioxus various attempts have 

 been made to further homologize the two animals. In these dis- 

 cussions one point has been made much of, especially by MacBride 

 '98, viz., the similar origin of the body cavities. He says: "The 

 mesoderm originates in Amphioxus as a series of true gut 



