124 r. M. BALIOUR. 



species of starfish will develope without impregnation in simple 

 sea water. The development is quite regular and normal though 

 much slower than in the case of impregnated eggs. It is not 

 definitely stated that polar cells are formed, but there can be no 

 doubt that this is implied. Professor Greeff's account is so pre- 

 cise and circumstantial that it is not easy to believe that any 

 error can have crept in ; but neither Hertwig nor Fol have been 

 able to repeat his experiments, and we may be permitted to wait 

 for further confirmation before absolutely accepting them. 



It is possible that the removal of part of the protoplasm of 

 the egg in the formation of the polar cells may i)e a secondary 

 process due to an attractive influence of the nucleus on the cell 

 protoplasm^ such as is ordinarily observed in cell division. 



Impregnation of the Ovum. 

 A far greater amount of certainty appears to me to have been 

 attained as to the effects of impregnation than as to the changes 

 of the germinal vesicle which precede this, and there appears, 

 moreover, to be a greater uniformity in the series of resulting 

 phenomena. For convenience I propose to reverse the order 

 hitherto adopted and to reserve the history of the literature and 

 my discussion of disputed points till after my general account. 

 FoPs paper on Asterias glacialis, is again my source of 

 information. The part of the germinal vesicle which remains in 

 the egg, after the formation of the second polar cell, becomes 

 converted into a number of small vesicles (Fig. 10), which aggre- 

 gate themselves into a single clear nucleus which gradually 

 travels toward the centre of the ^^2^ and around which as a 

 centre the protoplasm becomes radiately striated (Fig. 11). This 

 nucleus is known as the female pronucleus} In Asterais 

 glacialis the most favorable period for fecujidation is about an 

 hour after the formation of the female pronucleus. If at this 

 time the spermatozoa are allowed to come in contact with the 

 egg, their heads soon become enveloped in the investing muci- 

 laginous coat. A prominence, pointing towards the nearest 

 spermatozoon, now arises from the superficial layer of proto- 

 plasm of the egg and grows till it comes in contact with the 

 spermatozoon (Fig. 12 and 13). Under normal circumstances 

 the sj)ermatozoon, which meets the prominence, is the only one 

 concerned in the fertilisation, and it makes its way into the e^^^ 

 by passing through the prominence. The tail of the spermatozoa, 

 no longer motile, remains visible for some time after the head 

 has bored its way in, but its place is soon taken by a pale conical 

 body which is, however, probably in part a product of the meta- 



' Accordiiifi^ to llcrtwig's most rcceut slateiueul a nucleolus is present 

 in this nucleus. 



