BEHAVIOR OF THE GAMETES 237 



the formation of the perivitelline space is less precipitous. In the egg of the 

 bird (e.g., pigeon or hen) (fig. 126), a vitelline space filled with fluid appears 

 during the latter phase of oocyte growth in the ovary which separates the 

 surface ooplasm of the egg from the vitelline membrane. The egg is free to 

 revolve in this vitelline space. In the prototherian mammals, the zona pellucida 

 evidently functions in a manner similar to that of the bird or reptile (figs. 

 46, 127). However, in the metatherian and eutherian mammalia, the zona 

 pellucida becomes separated from the ooplasm of the egg's surface with the 

 subsequent development of a perivitelline space at fertilization or during early 

 cleavage (figs. 115, 118, 124, 125). 



It is to be observed, therefore, that there are two general groups of egg 

 or vitelline membranes in the phylum Chordata which assume an important 

 role at fertilization and during the earlier part of embryonic development: 



( 1 ) those membranes which become separated from the egg surface in a 

 somewhat dramatic manner at fertilization, and 



(2) membranes which separate gradually during the late phases of ovarian 

 development and during early embryonic development. 



In the former group are to be found the egg membranes of the eggs of 

 Amphioxus, teleost and many other fishes, and the amphibia; in the latter 

 group are the membranes of eggs of Styela, elasmobranch fishes, reptiles, birds, 

 and prototherian mammals. The higher mammalian eggs appear to occupy 

 an intermediate position. 



The separation of the so-called fertilization membrane has been most in- 

 tensively studied in certain invertebrate forms. As a matter of interest, some 

 of the processes involved in membrane elevation in various invertebrate eggs 

 are herewith described briefly. 



In the nematode, Ascaris, the egg exudes a jelly-like substance after the 

 sperm has entered. This substance hardens to form a thin, tough membrane 

 which later thickens and expands. The egg also appears to shrink, leaving an 

 enlarged perivitelline space between the egg surface and the outer hardened 

 membrane (figs. 128, 133,C-E). 



The formation of the fertilization membrane in Echinarachnius, a genus 

 of sea urchins, was the subject of intensive study by Just ('19). In this species 

 the egg is larger than that of the sea urchin, Arbacia. According to Just's 

 account, the fertilization membrane starts as a "blister" at the point of sperm 

 contact; from this area it spreads and rapidly becomes lifted off from the 

 general surface of the egg. Heilbrunn ('13) studied the fertilization membrane 

 of the sea urchin's egg before fertilization and describes it as a vitelline mem- 

 brane, "probably a gel or semi-gel" which is present at the surface of the 

 egg. It becomes visible as a distinct membrane when lifted off from the egg's 

 surface after fertilization. As this elevation occurs, according to Runnstrom, 

 cortical granules are exuded from the surface of the egg, accompanied by a 



