100 THE AMERICAN ARBACIA 



Psammechinus microtuberculatus, and small eggs from large species, e.g., 

 Tripneustes esculentus. Arbacia has a fairly small tgg (for sea urchins) 

 which cleaves at an intermediate rate. For sizes of eggs, see Classifica- 

 tion, Part I, Chapter 9. 



Fox (1938) has found that the cleavage rate of some species (Para- 

 centrotus lividus) is different in different localities. Horstadius (1925) 

 found that in the same locality, Naples, the cleavage rate of this species 

 differs in different seasons of the year ; winter eggs and summer eggs 

 have a different rate when kept at the same temperature. This is true 

 also, to a limited extent, o{ Arbacia punctulata ; the eggs obtained late in 

 the season (September) are slower to cleave, irrespective of temper- 

 ature, than those obtained at mid-season (July, August). 



d. Cleavage Without Membranes 



If the fertilization membranes are removed soon after fertilization, by 

 shaking, the eggs develop quite normally, though somewhat spread 

 out (Plate XVI, Photograph 7). They are held together by the hyaline 

 layer. They cleave at about the same rate but become free-swimming 

 earlier, since the fertilization membrane does not have to be dissolved 

 by the "hatching enzyme". Normal plutei are formed. 



If the fertilization membranes are removed and the eggs placed in 

 sea water without calcium, the hyaline layer does not form and the 

 cells are no longer held together. The isolated cells cleave several 

 times and among them, micromeres may be distinguished by their 

 size (Plate XVI, Photograph 9). Soon the isolated masses of cells go 

 to pieces. 



e. Sperm Entrance and Cleavage Planes 



A sperm may enter at any point on the surface of the egg, as noted by 

 many observers. This can be best demonstrated in centrifuged Arbacia 

 eggs, where it may be observed to enter in any zone of the stratified 

 egg, pigment, yolk, clear layer, or even the oil cap. 



The tail of the sperm has not been observed to enter the Arbacia tigg, 

 but is left outside, though the portion adjacent to the head is often 

 seen within the fertilization membrane, in the perivitelline space. This 

 portion as well as the distal end soon disappears as though dissolved. 

 The tail is left outside in the Lytechinus ( Toxopneustes ) tgg (Wilson's 

 Atlas 1895, p. 14), and probably in other sea urchins, as stated by 

 Wilson, more positively in the earlier editions of 77?^ Cell (Compare the 

 first edition 1896, p. 136, 149 and second edition 191 1, p. 188, 200 



