MACKEREL INVESTIGATIONS IN 1897. 9 



the spermatozoa or the development around the vitellus of a water 

 space which the spermatozoa can not traverse. Impregnation in the case 

 of the mackerel is naturally and easily accomplished by the intimate 

 mixture, in a small quantity of sea water, of the freshly stripped 

 ova with a small amount of perfectly ripe sperm from a previously 

 selected male. The objection to the adoption of the dry method lies in 

 this: Owing to its greater weight the micropyle, which is the only 

 point through which a spermatozoan can enter the egg, lies always at 

 its lowermost pole. Now, when the eggs are only moist and the sperm 

 is poured over them, a much greater quantity is needed, and more of it 

 will adhere to the membranes than when the eggs are freely suspended 

 and the micropyle exposed to the active swimming spermatozoa. The 

 dry method may be just as effective as the wet, but it can not be more 

 so, and has the slight disadvantage just mentioned. 



Simultaneously with the progress of the internal act of fusion of the 

 male and female pronuclei, other and more obvious external changes in 

 the egg are taking place. The protoplasmic pellicle, which at first 

 forms a layer of uniform thickness, begins to accumulate with a peculiar 

 wave-like streaming at the lower or micropylar pole of the egg. At 

 first the thickening thus produced is in the form of a scarcely evident 

 meniscus, which passes off on all sides into the thin pellicle. Gradually 

 it increases in thickness until nearly all of the protoplasm has accu 

 mulated in a sharply defined disc, the blastodisc, which, when com- 

 pleted, has a circular outline. Its outer surface is in contact with the 

 egg-membrane and conforms very nearly to its curvature. The inner 

 surface of the blastodisc has a strongly convex, almost conical, form 

 and projects into the yolk. At its margins, which rise abruptly from 

 the yolk, the blastodisc passes into continuity on all sides with the 

 protoplasmic pellicle, which has now become reduced to an excessively 

 thin and delicate, but perfectly intact, layer. The completed blasto- 

 disc becomes the seat of future developmental processes. The remain- 

 der of the protoplasmic pellicle takes but little part in these processes, 

 and is known as the periblast layer. Oriented with reference to the 

 micropyle, the blastodisc seems always to be a little eccentric, and the 

 micropylar thickening is readily found just outside of its center. The 

 perivitelline space, which is described above as an exceedingly shallow 

 space between the egg-membrane and the mature vitellus, has now, 

 owing to the change in form of the latter, altered its character. Owing 

 to the contact of the greater part of the external surface of the yolk 

 and of the blastodisc with the egg membrane, it is in these regions 

 obliterated or so exceedingly shallow as to escape detection. But 

 there is a deep ring-like space running all around the margin of the 

 blastodisc and corresponding with a circular groove which forms the 

 boundary between the yolk and disc. 



The pellicle of protoplasm enveloping the oil-drop has, like that sur- 

 rounding the yolk, collected in a bleb-like mass on the under surface of 

 the oil. Its shape is similar to the blastodisc except that it is not con- 

 strained within a limiting membrane and has conformed to the charac- 



