EGGS AND LARVAE OF TELEOSTEANS. 131 



the rhythm of segmentation. At the stage shown in Plate III. fig. 3, one and 

 a half hours after fertilisation, the central part of the protaplasmic disc is much 

 the thickest, forming a somewhat conical protuberance downwards into the 

 yolk. The protuberance afterwards disappears, and at the end of three hours 

 the blastodisc has the shape shown in Plate III. fig. 4, the lower surface being 

 uniform. Then a second aggregation of the protoplasm begins, but this time 

 towards two points, as shown in Plate III. fig. 5, producing the first division of 

 the blastodisc. Thus the aggregation of the protoplasm towards the micro- 

 pylar pole may be regarded as a contraction towards one central point, and 

 the first division as due to a contraction towards each of two separate points. 



The expulsion of a polar body through the micropyle I observed repeatedly 

 in the ova of Pleuronectes cynoglossus studied at Millport last June. It took 

 place in both fertilised and unfertilised ova. But I was unable to discover 

 either in the living eggs, or in the fresh eggs treated with reagents on the slide, 

 any nuclear spindle either before or during the expulsion of the polar body. In 

 one or two instances I noticed a minute pyriform projection of protoplasm on 

 the surface of the blastodisc after the latter had withdrawn itself from the 

 vitelline membrane (Plate III. fig. 7) This might be either a second polar body, 

 or simply the proximal part of the first drawn away with the receding blastodisc, 

 from the inner end of the micropyle. Agassiz and Whitman, in Pelagic Stages, 

 p. 19, mention the formation of two polar bodies in Teleostean ova, and under- 

 take to describe them in a subsequent memoir. 



My results, as far as they go, concerning the unfertilised ova, are in agreement 

 with those of Hoffmann. In the unfertilised ovum, as a rule, the expulsion of 

 the polar globule through the micropyle and the concentration of the protoplasm 

 take place just as in the fertilised ovum, with the exception that the latter 

 process goes on much more slowly in the unfertilised ovum. I have never seen 

 any traces of segmentation in the unfertilised ovum. The small proto- 

 plasmic body on the blastodisc seen two hours after fertilisation, and shown in 

 PI. III. fig. 7, was seen also at the same stage in the unfertilised ovum. The 

 aggregation of the protoplasm in the fertilised ovum is finished about three 

 hours after fertilisation, and the stage shown in PI. III. fig. 4, is reached. At 

 this stage the unfertilised ovum stops unchanged, being perfectly incapable of 

 segmenting. PI. III. fig. 9 shows an unfertilised ovum of PI. cynoglossus six 

 hours after shedding, at which time the fertilised ova were in the eight-cell 

 stage, and the cells again dividing to form the sixteen-cell stage. The 

 unfertilised ova were in the condition shown in PI. III. fig. 9, twenty- four hours 

 after being shed, and remained unchanged till they died. I was not able to 

 determine with absolute certainty at what stage the spermatozoon entered the 

 fertilised ovum. This occurs, as is evident from PI. IV. fig. 2, during the first 

 half hour, and I am inclined to believe that it takes place immediately the ripe 



