THE EARLY HISTORY OF THE OVUM. 20/ 



portions are sharply distinct, the protoplasm resting as a small 

 cap upon the large sphere of pure food yolk. 



The egg is surrounded by primary and secondary envelopes, 

 the former arising before the ovum has escaped from the ovary, 

 the latter from the ovarian ducts. In the vertebrates the pri- 

 mary envelopes are at most three in number. These are, from 

 without inwards, (i), a vitelline membrane, structureless in. 

 character ; (2), a zona radiata (or zona pellucida) traversed by 

 minute pores ; and (3), a thin and delicate inner membrane. 

 These are not constant, and any one or two may be lacking 

 in a given egg. In some cases (teleosts and Petromyzon) aa 

 opening (micropyle) exists, through which the spermatozoon 

 obtains entrance to the egg. 



Of the secondary envelopes one of the simplest conditions 

 is found in Petromyzon, where the outer surface of the egg is 

 covered with a thin layer of adhesive mucus, which serves to 

 fasten the egg to stones, etc. In the myxinoids the egg envel- 

 ope is more horny, and is provided at either end with anchoring 

 hooks. The descriptions would also imply that at the time of 

 laying there was an outer sheathing capsule.-' In the amphibia 

 the eggs receive a coating in their passage down the oviduct 

 which swells into a jelly when in contact with water. In the 

 elasmobranchs the eggs are enclosed in a horny capsule, usually 

 quadrangular in outline, while in the reptiles and monotremes 

 the oviducts secrete around each egg a calcareous shell. 



The birds present the most complicated condition. Here 

 the eggs, after they have entered the oviduct, receive first a 

 layer of albumen (the 'white'), a portion of which, firmer than 

 the rest, is twisted into a spiral chalaza at either end. Outside 

 of this there is next deposited a double shell membrane, and 

 then, by the next division of the duct, the calcareous shell 

 (Fig. 212). 



The spermatozoa arise in the canaliculi seminiferi of the 

 testes (p. 126), but they present many differences from the eggs. 

 These in merest outline are as follows : In every cell of the 



1 No one has yet described the origin of these envelopes of the cyclostome egg ; it may 

 be that they are ovarian in origin, a view which seems the more probable from the absence 

 of oviducts. 



