106 CRUSTACEA. 



to the limbs of the abdominal segments (pleopoda) by nieans of the 

 secretion of a special cement-gland. 



2. Cleavage and Formation of the Blastoderm. 



The Crustacean egg is, as a rule, distinguished by the laige 

 amount of food-yolk contained in it. The latter consists of spherical 

 particles interspersed with fat-drops. In most cases the food-yolk 

 is found equally distributed throughout the egg, although, as a rule, 

 the yolk-spherules are of smaller size at the surface of the egg. In 

 a few cases, in eggs that contain less nutritive yolk, a superficial 

 layer of protoplasm (formative yolk) is developed (e.g., many 

 Cladocera and CetocTiilus). As a rule, part of the formative 

 yolk is evenly distributed between the particles of food-yolk, while 

 the rest is massed near the first cleavage -nucleus. Rarely, a& 

 in Moina, the polar differentiation of the egg is made evident 

 by the unequal distribution of the food-yolk, which accumulates 

 in the vegetative half of the egg. In Moina also, the first 

 cleavage-nucleus is found (as also in Cetochiius) not exactly in the 

 centre of tlie egg, but in an excentric position, somewhat nearer 

 the animal pole. The first cleavage -nucleus usually lies, together 

 with an accumulation of protoplasm, near the centre of the egg ; 

 and even in those forms (e.g., Mysis) in which discoidal cleavage 

 takes place, it originally occupies a similar position. 



The Crustacean egg, after the ejection of the polar bodies and 

 subsequent fertilisation, is usually at hrst surrounded by a homo- 

 geneous cuticular envelope, Avhich is probably secreted by the egg 

 itself, and must therefore be called the vitelline memhrane. 



It is not as yet luiiversally admitted that vitelline membrane is the correct 

 designation for this envelope. The formation of this membrane takes place either 

 in the lower portion of the oviduct, or only after the egg is laid (fertilisation 

 occurring simultaneously). Claus considered that it arose as a secretion or 

 hardening of the external layer of the yolk, and therefore assumed that it was 

 a vitelline membrane, while Van Beneden (No. 1) thought it probable that it 

 originated from the cells of the follicles or from the epithelium of the oviduct 

 (in those cases wliere no follicles are developed), and accordingly called it the 

 chorion. This latter name has been adhered to by many recent authors. 

 H. Blanc (Xo. 35), in support of this view, has shown that in Cuma the 

 membrane adlieres more closely to the follicle-cells than to the surface of the 

 egg. The view that it is a vitelline membrane, -whicli Ludwig also held, 

 receives its strongest support from the observations of Claus, who was able 

 to prove by means of measurements that, in Chondr acanthus, a decrease in 

 the volume of the egg took place simultaneously with its appearance, and of 

 Gkobben, who showed that in Ccfochilus (No. 21) this membrane is formed 

 after the egg is laid, when a similar contraction of the egg takes place. These 



