ON THE DEVELOPMENT OF PARASITIC COPEPODS. 73 



between slide and cover glass and therefore in the same plane 

 (Fig. 4, -B). Thus the first cleavage obeys Hertwig's rule 

 whether the compression be applied during or after the meta- 

 phase and possibly during the ensuing resting stage. 



Hertwig says that if frog eggs are thus compressed normal 

 embryos will develop, although a totally different distribution of 

 nuclei results. 



Born found that if a frog's egg were inverted before formation 

 of first cleavage spindle the relative density of protoplasm and 

 yolk would cause streaming movements in the egg, the proto- 

 plasm and nucleus rising through the yolk to the upper pole. 

 But it is probable that these streaming movements would be 

 hindered by the astral rays after formation of the spindle. 



In the parasitic copepods the direction of many of the spindles 

 is influenced by the pressure, and Hertwig's law applies in most 

 cases. But the peculiar form of cleavage seems well adapted to 

 variations in pressure. The blastoderm lying on the yolk may 

 be compared to a rubber bag divided by lines into polygonal 

 areas. The bag may be pressed into various shapes without 

 altering the mutual relation of adjacent polygons. The only cell 

 whose form is changed very much by pressure is the yolk cell. 



I have said that the distal egg (the one at the free end of the 

 egg string) is larger than the others. This is due to the fact 

 that in the oviduct it presented more surface for absorption of 

 nutriment through the wall of the oviduct. While the other 

 eggs present only a thin edge toward the source of food, this 

 last egg of the series presents this edge and one whole flat side 

 in addition. Usually it does not remain flat, but becomes more 

 or less hemispherical on the free side while still in the oviduct. 



The cytoplasm of many cells is formed in large part from the 

 substances that escape from the nucleus at the first maturation 

 and early cleavage divisions. Dr. Conklin traced a similar proc- 

 ess in gasteropods and in the living eggs of ascidians, and it may 

 be a general phenomenon. In other words, a quantity of chro- 

 matin is dissolved and escapes into the cytoplasm in many and 

 perhaps all cell divisions. 



In Paramcecium the macronucleus and a large part of the 

 substance of the micron ucleus escapes into the cytoplasm at each 

 copulation and may constitute necessary ingredients of the cyto- 



