196 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 39. 



together and are not separated one froni another by secondary walls 

 as in the Caligidae. But the same free space around each egg, neces- 

 sary to secure the requisite supply of oxygen for the developing 

 enabryo, is obtained by the separation of the two membranes cover- 

 ing the egg. The stiff outer membrane, which ordinarily fuses with 

 the vitelline membrane into a honeycomb substance, here remains 

 separate, and swelling a little after entering the external sack, 

 maintains a narrow space for aeration around the egg. The loose 

 manner in which the eggs are packed inside the external sacks favors 

 such a method of aeration. Instead of being arranged in a single 

 row pressed tightly together and strongly flattened at right angles 

 to the long axis of the sack, they are thrown in loosely without any 

 definite arrangement. The former conditions which prevail in the 

 Caligidse, Dichelestiidae, and Lernseidse necessitate the presence of 

 secondary partitions to prevent the embryos from being smothered. 

 But the latter condition, which is common to free swimming forms 

 and to the Ergasilidse and Chondracanthidae as well as the present 

 family, can dispense with these partitions, the stiff shell membrane 

 serving amply to keep the eggs properly aerated. 



EMBRYONIC DEVELOPMENT WITHIN THE EGG. 



ARRANGEMENT OF EMBRYOS. 



Owing to the lack of any definite arrangement of the eggs within 

 the external sacks, there is a corresponding absence of regularity in 

 the position of the embryos. 



In the Caligidse the germinal area is found in the center of the 

 proximal side of each of the flattened eggs, and the longitudinal 

 axes of the developing embryos are closely parallel. 



The only general statement that can be made with reference to the 

 present species is that development usually begins at that pole of the 

 egg which is diametrically farthest away from the external surface of 

 the egg sack. This point afterwards becomes the posterior end of the 

 enbryo so that, when fully developed as well as during development, 

 the larvse lie in the egg sacks with their heads toward its outer surface; 

 that is, they are arranged radially. 



Further than this there seems to be nothing definite, since the ger- 

 minal area spreads with equal impartiality over any of the various 

 surfaces toward the egg sack. And in those eggs which lie near the 

 center of the sack and entirely surrounded by other eggs, the point 

 where development begins may be turned in any direction indis- 

 criminately. 



CLEAVAGE STAGES. * 



Segmentation is purely superficial, proceeding gradually from the 

 point just mentioned over the whole surface of the egg. Beneden 

 has stated that in Anchorella, Lernseopoda, and Brachiella, which are 



I 



