10 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. 



ter of the fluids between whicli it lies, like tlie drop of water which 

 gravitates to the under side of a soap bubble. This smaller disc takes 

 no important part in development and undergoes no change until it 

 becomes fused with the surrounding blastodisc. It will not be further 

 considered. 



Although just described as accompanying and succeeding fertiliza- 

 tion, these processes must not be considered wholly as a consequence 

 and therefore as a test of fertilization, for even in the absence of sper- 

 matozoa, changes almost precisely similar and likewise resulting in the 

 formation of a blastodisc ensue, but are much later in reaching their 

 fulfillment. In this case development never, so far as I have observed, 

 proceeds any further, and in the course of a day or two the vitellus 

 contracts, turns opaque, and rapidly disintegrates, the egg usually 

 sinking to the bottom soon after the contraction becomes evident. 



The brief chronological index to the stages of embryological develop- 

 ment which follows is founded upon the time record of a series of 

 embryos which developed at a mean temperature of 12.9° C. After 

 the blastodisc has attained its greatest thickness and internal convexity, 

 which occurs 1 hour and 20 minutes after impregnation, the heaped-up 

 protoplasm slowly subsides, flattens a little, becomes slightly elongated 

 along one axis, and remains quiescent for a time in this condition. 



At 1 hour and 40 minutes after imj)regnation, the first segmentation 

 furrow begins to appear as a narrow transverse depression across the 

 middle of the outer surface of the disc, while on the inner surface 

 appears a button-like elevation which is quickly subdivided by an 

 internal depression placed exactly opposite to the external one. The 

 halves of the button then subside and diverge, and, moving with a 

 wave-like motion outward on each side of the internal furrow, cause a 

 pair of prominent elevations to api^ear on this surface of the blastodisc. 

 The external furrow rapidly extends to the margins of the blastodisc 

 and cuts deeper into the protoplasm, which thus becomes almost sepa- 

 rated into two equal halves or blastomeres, in each of which the proto- 

 plasm becomes actively heaped up. At the same time each blastomere 

 tends to become circular, so that the blastodisc elongates in tbe axis 

 at right angles to the first furrow. The segmentation furrow never 

 cuts quite through the disc, but spares a thin layer of protoplasm on 

 its deeper surface, so that the otherwise separate blastomeres remain 

 connected by a protoplasmic web in contact with the yolk. The active 

 process of division is completed very quickly. The heaped-up blasto- 

 meres then subside and pass into a resting state, during which the 

 segmentation furrow becomes much less conspicuous. This condition 

 is of relatively long duration, and it is interesting to note here that 

 the slower development of this species as compared with the Spanish 

 mackerel is due almost entirely to the greater duration of the resting 

 periods, the periods of activity in each being of almost equal duration. 



The second furrow, formed at right angles to the first, is completed 

 in 2 hours and 35 minutes after the mixing of the ova and the sperm, 



