BULLETIN OF THE UNITED STATES FISH COMMISSION. 145 



the fact that most of the natm^alists who have made the study of fish 

 embryos a specialty hiWe had the ill fortune to have the chance to 

 watch only a part of the developmental stages. 



THE IMPREGNATION OF THE EGG. 



Upon this subject there are very few reliable observations. As Axel 

 Boeck truly remarks, the micropyle is often diificult to find; and what 

 makes the matter still more troublesome is the size of the egg, which 

 makes it necessary to use lenses of long working distance, and to am- 

 plify with high power eye-pieces. To get an egg into position is not 

 unfrequently a difficult performance, and by the time everything is 

 arranged for observation impregnation has been effected and your efforts 

 are wasted. It is doubtless correct to say that a single spermatozoan 

 is eflective in the fertilization of an egg. I have frequently found a 

 number of dead ones sticking fast by their heads to the egg-membrane 

 near the micropyle, but I have never witnessed their actual entry, 

 although 1 have frequently made attempts to see the phenomenon, but 

 so far without success. From all that we can learn it is undoubtedly 

 true that the presence of spermatozoa with freshly laid uuimpregnated 

 ova at once tends to cause them to absorb water, as is well known to 

 every practical spawn-taker. That their presence in the egg exerts a 

 great influence on the rapid formation of the germinal disk in the her- 

 ring, shad, and cod is equally certain. What the exact nature of the 

 changes may be that are first of all induced by the presence of the 

 spermatozoan in the egg of the Teleostean fish, we are not yet prepared 

 to say. Most if not all the most satisfactory observations upon the phe- 

 nomena of imi)regnation have been conducted on the very much more 

 miuute ova of invertebrates, where it has evidently been miuch easier 

 to see the process and follow it in detail. Its eifects are soon visible, 

 however, as the remarkable phenomenon of segmentation which begins 

 soon after fertilization has been effected. 



SEGMENTATION OF THE GERMINAL DISK. 



The segmentation of the germinal disk of the mackerel is essentially 

 similar to that of the cod. The first cleavage is transverse, resulting in 

 two cells. The next segmentation is at right angles to the first, and, 

 when completed, divides the two cells of the first cleavage into four ; 

 the next cleavage is in a direction at right angles to the last and results 

 in the formatioii of eight cells. Beyond this point the cleavage becomes 

 more or less irregular, except that the germinal disk remains for a con- 

 siderable time composed of a single stratum of cells, as shown in Fig. 

 3, one hour and forty minutes after impregnation. The rhythmical 

 nature of the process of segmentation up to this time has already been 

 alluded to, and it no doubt continues, but the cells soon become too 

 smg-ll to be followed up so as to observe the intervals of rest and activ- 

 Bull. U. S. F. C, 81 10 



