334 THE EMBRYOLOGY OF THE li LOW-FLY IN THE EGG. 



developed, but this is the hiatus which has not hitherto been 

 bridged over by observation. No one has actually seen the 

 process by which these are converted into epithelial elements. 



The white specks without nuclei, which Korotneff speaks of, 

 have long been observed. Zaddach [96] described them as a 

 number of clear vesicular spots, and Leuckart [20] says of 

 them : ' I must hold it as an error when it is maintained that 

 they are nucleated cells, and therefore the same which ulti- 

 mately form the blastoderm. These clear flecks are present 

 only in small numbers, and are separated from each other by 

 considerable spaces ; they are transparent, without nuclei, and 

 with a doubtful investing membrane, so that they may easily be 

 taken for drops of sarcode.' And he repeats : ' The clear 

 flecks which appear after fertilisation in insects' eggs, and 

 without exception in the peripheral substance of the yelk, are 

 not cells, but rather bodies which lead to cell formation.' 

 That Leuckart in 1858 should have held such a view is not 

 astonishing, but that it should still be entertained seriously 

 is incomprehensible to me. The production of a cell from a 

 clear speck without a nucleus, a mere vacuole, and possibly a 

 post-mortem appearance, is at variance with all that is known 

 regarding cell-formation, and I should have thought identical 

 with the spontaneous generation of cells. 



The presence of stellate cells within the yelk in Arthropods, 

 which multiply with great rapidity before the blastoderm 

 makes its appearance, is an undoubted fact, but the supposition 

 that they are formed from the female pronucleus, or from the 

 germinal vesicle, and that they ultimately become the epithelial 

 blastoderm, is a theory which, in my opinion, is not supported 

 either by what happens in other forms of animal life, or by any 

 direct observations. Precisely similar cells make their appear- 

 ance in the interior of the fat bodies, and result from the 

 division and subdivision of the original cells in which the fat is 

 deposited (see Chapter VIII.), and I have long held that the 

 centrolecithal cells are mesamceboid or parablast cells. 



The Parablast— Until (luite recently all the tissues of the 

 body in every form of the Metazoa were supposed to originate 



