OF THE ROTATOKJA. 445 



seems to serve the purpose of a uterus, and has received the name of ovisac. 

 The orifice of this oviduct or ovisac into the cloaca is called the " vaginal 

 orifice ; '' the vaginal sheath, spoken of by Ehrenberg, would appear to be 

 either the termination of the oviduct or sometimes the cloaca itself. The 

 o\iduct may occasionally be deficient. Prof. Huxley states that he could 

 discover no such passage in Lacinularia. 



The egg, having descended into the cloaca, is expelled thence by means of 

 a strong contraction of the whole body, and, in the act of escaping, involves 

 the eversion of the cloaca. The time occupied in the formation of the sum- 

 mer egg, from its first appearance as a vesicular space in the ovary to its 

 completion and extnision, is very brief, generally only a few hours. 



AYe have noted the discharge of several eggs adherent together, and their 

 subsequent attachment at the anal outlet. In other Rotatoria, likewise, ova 

 expelled singly attach themselves at the posterior extremity of the body, 

 singly or united together by a gelatinous matter, and not uncommonly at- 

 tached by evident cords or pedicles to the parent. This is seen in Megalo- 

 trocha, in Brachionus ruhens, and in B. Pala. 



In the species last named, as many as ten may often be seen in a group 

 near the cloacal orifice. In AscomorpTia, some six may be found adherent. 

 In Polyarthra (XXXVIII. 30), not more than one egg is found attached at the 

 same time. Thus the eggs are carried about by the parent one after another, 

 arriving at maturity and escaping from its shell. The like phenomenon is seen 

 in various Entomostraca, and in Polynoe, Exogona, and other Vermes, which 

 likewise produce both summer and winter ova. Among the urceolate Ro- 

 tifers, the eggs escape into the case or gelatinous investment, and there pro- 

 ceed to their ultimate development, safe from many obnoxious infiuences and 

 from destruction by other animals. 



Development of the Embryo. — The following changes transpire pre- 

 paratory to the construction of the embryo. The nucleus is seen to elongate, 

 and then to present a constriction about its middle (XXXVII. 5) ; the yelk 

 at the same time shows a similar constriction, which continues to deepen in 

 correspondence with that of the nucleus, until at length there are two seg- 

 ments, each with its contained nucleus — the result of the fission of the primary 

 one. Leydig states that this division is not into two equal portions, but that 

 a segment is cut off from one end or pole (XXXVII. 2 h), and that in the 

 continued segmentation which ensues, this same unequal fission is again 

 and again repeated. However this may be, the act of division goes on 

 (XXXVII. 8) until at length the whole yelk is broken up into a mass of 

 minute cells, and its opacity increased by the number of molecules they con- 

 tain (XXXVII. 2 c, g). Out of this mass, the tissues and organs of the em- 

 bryo are developed, appearing in their characteristic forms without any, or 

 otherwise very slight, transitional phases (XXXVII. 2e,d; XXXVIII. 9). 

 It is characteristic also of Rotatoria, in common with all the Vermes, that 

 the embryo is generated from the entire yelk, and not, as in Crustacea and 

 stiU higher animals, from an accessory body superposed upon the yelk, into 

 which the yelk is gradually taken up. Dr. Carpenter remarks that the mode 

 of development is in all essential respects the same as that of the Xematoid 

 Entozoa, each group of ceUs evolving some one principal organ. 



The order of succession of the parts of the embryo in the egg is thus de- 

 scribed by Ehrenberg in the instance of the Megalotrocha alho-flavicans : — 

 " A turbid central spot appears, which becomes the oesophageal bulb and 

 teeth ; a blackish granular oval body is also seen posteriorly ; the eyes gra- 

 dually become red, and a motion of the cilia of the head is visible ; after some 

 hours the whole foetiLs, which is folded up, turns itself round, the shell bursts, 



