^So WILSON. [Vol. VIII. 



Part I. — Normal Development. 



As is well-known, the cleavage of AmpJiioxns after the 

 4-celled stage is distinctly unequal, and I shall in general use 

 the terms macromeres and micromeres to designate the larger 

 and smaller blastomeres, respectively. These terms are, how- 

 ever, used solely for the sake of convenience and imply nothing 

 as to the ultimate fate of the blastomeres. 



I. The first and second cleavages take place precisely in 

 the manner described by Hatschek, dividing the egg- into four 

 equal parts (Plate XXIX, Figs, i, 2, 3). Even in the 4-celled 

 stage slight variations exist in the position of the blastomeres. 



which a small quantity of freshly discharged spermatozoa had been added. By 

 this method the eggs may be obtained perfectly clean and in any desired quantity. 

 The embryos were preserved by a great variety of reagents, the best results being 

 given by Lang's fluid (sublimate-acetic), various combinations of picric acid (picro- 

 sulphuric, picro-acetic, picro-sublimate), and the Fol-Flemming chrom-osmium- 

 acetic fluid. Perfect temporary preparation of the early stages is afforded by the 

 use of the strong glycerine-acetic mixture (^ glycerine, >^ water, Y^ glacial acetic) 

 that I have found of such essential service in the study of annelid development. 

 Careful comparisons of preparations thus made with living embryos and with 

 balsam preparations of picro-sulphuric, subUmate-acetic, etc., specimens show that 

 with proper precautions the only perceptible alteration is a very slight swelling, 

 and even this does not take place if the ingredients of the mixture are used in 

 exactly the right proportion. 



The eggs are always laid in the late afternoon, as a rule between five and six 

 o'clock. There is a marked periodicity in the discharge of the reproductive ele- 

 ments, spawning-periods of two or three days alternating irregularly with quiescent 

 periods of about the same length, during which no reproductive elements are 

 discharged even in favorable weather. The beginning of a spawning-period is 

 indicated by the accumulation of masses of ova or spermatozoa lying free in the 

 anterior median ventral portion of the atrial chamber, where they can easily be 

 seen in the living animal through the transparent atrial walls. Almost invariably 

 the males are the first to discharge, and the clouds of spermatozoa set free in the 

 water seem to stimulate the females to spawn. The reproductive elements are 

 very rapidly discharged, passing in a steady narrow stream along the median ven- 

 tral line of the atrial chamber and out to the exterior t/iroiigh the atrial pore. 

 I have observed this process in hundreds of animals and have never in a single 

 case seen a discharge through the mouth. My observations on this point, which 

 were made with particular attention and care, agree with the original account of 

 Quatrefages (22), afterwards confirmed by Willey {34), and differ from those of 

 Kowalevsky (17) and Hatschek (10). 



It is difiicult to believe that Hatschek's very explicit statements (Amphioxus, 

 p. 141) rest upon erroneous observation, but I can suggest no other explanation 

 of the contradiction unless the animals vary in habits from year to year. 



