105 



used and the mode of proceeding he has employed ; and his results 

 show that he has successfully surmounted the obstacles to micro- 

 scopic investigation caused by the opacity, the great size, and the 

 tendency to movement inherent in the egg. 



The fact of the impregnation of the ovum through the entrance 

 of the spermatozoon into the yelk by its own movement was com- 

 municated to the Royal Society in a preceding paper*, and the ori- 

 ginal experiments there referred to as serving to establish the fact, 

 are now detailed. In addition, the circumstances affecting the pas- 

 sage of the sperm-body through the thick investing envelopes are 

 considered, and thence it is concluded, that " when there is any de- 

 ficiency in the usual power, arising from an unhealthy condition of 

 the fertilising body, or an increase in the resistance of thte yelk mem- 

 branes, the spermatozoon is unable to pass through the membranes 

 into the yelk and the egg remains unfertilized." 



The two small rounded bodies that appear on the surface of the 

 yelk in the interval or chamber between it and the investing mem- 

 brane, have been traced from their origin, through their changes, 

 till their disappearance after the equatorial division of the yelk. The 

 investigations as to the true import of these bodies have not been 

 further carried out, in consequence of the untimely death of the 

 author ; but his observations have induced him to put forth the fol- 

 lowing statement regarding them, viz. " that they are usually, and 

 perhaps invariably, at that part of the yelk at which the head of the 

 embryo is afterwards found." 



By following the changes in the segmenting yelk, evidence has 

 been obtained of the derivation of different parts of the future being 

 from definite segments of the yelk. Thus it has been found, that 

 the half of the yelk on one side of the second or crucial cleft begins 

 its subdivisions sooner than the opposite, and that the trunk and 

 tail of the embryo are derived from this first subdividing part, whilst 

 the head is produced from the other half. 



Having ascertained so much respecting the foundation of different 

 parts of the embryo, the author next determined that the axis or 

 spine will primarily lie in a line with the first cleft of the yelk, 

 though it may afterwards deviate somewhat from that line during 

 the growth of the embryo. 



* Philosophical Transactions for 1853, p. 271. 



M 2 



