114 J. Thomas Patterson. 



tioiij while two are associated with a rudimeutary accessory cleavage 

 furrow. The furrow was clearly visible in the living egg, where 

 it appeared as a shallow groove. In section it is characterized by 

 having a broad shallow depression, at the bottom of which is found 

 a distinct membrane plate of exactly the same appearance as that of 

 an early primary cleavage furrow, and differs from the latter only 

 in the absence of a membrane (Fig. 19, a. f.). It is, therefore, 

 rudimentary, and the presence of two supernumerary sperm nuclei 

 in its immediate vicinity leaves no doubt as to the interpretation 

 \ that it is an accessory furrow. One of the nuclei (Fig. 19, s. p., 



nucleus on right) has passed down into the coarser granular yolk 

 and undergone almost complete fragmentation, and this may be 

 the reason the furrow never proceeds to the point of forming a mem- 

 brane. 



In addition to the rudimentary cleavage this egg also presents 

 an interesting case of horizontal accessory cleavage (Fig. 4, c). 

 In reality this is not a cell division, because there is associated 

 with the cleft but a single nucleus, which lies just below the cleft. 

 Such cases would seem to be attempts at cell formation without 

 the accompanying nuclear division. They are not of any great im- 

 portance, as but two examples have been found. 



So far the writer has not observed the accessory cleavage after 

 the ten-celled stage, and in all probability it completely disappears 

 shortly after this period. It seems highly jDrobable that many of 

 the sperm nuclei degenerate soon after their entrance into the egg. 

 The degeneration occurs when they pass down into the coarse granu- 

 lar yolk; but so long as they remain superficially situated they seem 

 to possess the power of migration. Undoubtedly, there are not a 

 few eggs in which all of these nuclei degenerate before reaching the 

 margin, and hence such eggs would at no time show an accessory 

 cleavage. The writer does not believe, however, that this would 

 account for the failure of previous investigators to discover the 

 accessory cleavage of the hen's egg. Only a few of them have de- 

 scribed the four-celled stage, and, so far as we are aware, none have 

 figured the eight. The brief period during which they would most 

 likely observe this peculiar form of cleavage is, therefore, the one 

 to which they have given least attention. Furthermore, the acces- 



