MACKEREL INVESTIGATIONS IN 1897. 15 



to 24 Lours after impregnation, when embryological development had 

 reached the late blastnla and early gastrula stage. When, during the 

 early part of the third day of development, the body of the embryo is 

 well formed, with from 3 to 8 myotomes differentiated, there is a marked 

 increase in the specific gravity of the eggs. They then begin to sink 

 slowly toward the bottom, most of them remaining for a long time in 

 suspension, and a few (the number of which varies with the batch of 

 eggs and the density of the water) retain their position at the surface. 

 The cause of this increase in weight becomes more apparent as develop- 

 ment proceeds. In the first place, it is to be noted that at about this 

 time the oil-drop has been inclosed by the spreading blastoderm, and 

 already on the third day shows signs of absorption in a llattening of the 

 inner surface of the sphere and a diminution in size, shown by careful 

 measurements. The oil-sphere, which at first measures about 0.29 mm. 

 in diameter, has by the time of hatching decreased in one diameter to 

 0.16 mm., the other remaining at about its original size. The absorption 

 of the yolk and oil-drop and the building-up of the more compact 

 body of the embryo has, in the meantime, resulted in a contraction 

 of the vitellus and an increase in the size of the perivitelline water 

 space. The egg-sphere, therefore, while undergoing no increase in 

 size, has come to include relatively more solid matter, and is therefore 

 heavier. 



I have gone somewhat fully into this matter, as I desire to show that 

 the increase in sijeciflc gravity at tJiis time and subsequently is a 

 I)erfectly normal process. Probably any egg, the specific gravity of 

 which is so nearly that of the density of the water in which it is 

 developed — as was the case with these mackerel eggs — would undergo 

 a similar submergence during the later phases of development. The 

 eggs are not dead when they sink to the bottom (though of course 

 those which have died will also sink), but lie among the mass of debris 

 and sooner or later succumb to the influence of the decaying organic 

 matter. This subject, with its probable significance and practical 

 bearings, is considered in a later part of this report. 



2. RESULTS OF SUEFACE TOWINGS. 



It was early in the course of the investigation considered that 

 important aid might be rendered toward the jjractical ends of the work 

 if the distribution of mackerel eggs deposited naturally in these waters 

 could be determined, together with the environmental conditions under 

 which they were obtained. To secure this information, surface towinga 

 were begun on June 26 and continued daily, except during the progress 

 of two heavy storms, until July 27. After the arrival of Mr. Brett on 

 July 1, the collection of this material and full physical and meteorological 

 data bearing upon it were placed in his charge. Extensive and sys- 

 tematic examinations were made of the waters, especially about the 

 eastern entrance of the bay, and less fully elsewhere. The material 

 was fully examined by me, with special regard to the number and 

 condition of the mackerel eggfi present. The results were in part 



