172 ON THE STRUCTURE AND 



central oil-globule. They appear to me to be mesodermic cells, 

 analogous to the fatty rete of insects ; and, like them, they are 

 more abundant in the earlier stages than in the adult, and are 

 largely utilized in the elaboration of the structures which specially 

 characterise the adult, where they become consequently less con- 

 spicuous. 



A primary separation of the head from the body can be seen 

 even before the embryo quits the egg, which it now does, the cast 

 vitelline membrane being frequently seen, together with the 

 embryos, in the receptacle. The emlDryos (Fig. 17) are now of a 

 flattened or depressed form, strikingly in contrast with the com- 

 pressed condition of the adult, the diameter from the dorsal to the 

 ventral surfaces being much less than that from side to side. In 

 the centre is seen the large oil-globule, and the green cells by 

 which it is surrounded. The embryos are enveloped at this time 

 in a delicate skin [sk., Fig. 17), of which Sir John Lubbock says 

 that as it was not present when the egg was laid, it must have been 

 formed since, and he draws the conclusion that the young 

 Daphniae, so far from undergoing no metamorphosis, do in fact 

 enter the world in a very rudimentary condition, and that only 

 after the first change of skin do they assume the distinctive 

 characters of the genus. This skin is subsequently shed, but 

 previously to that we perceive the rudiments of the anterior limbs, 

 viz., the two pairs of antennae, the mandibles, and the first pair of 

 maxillae, all in the form of rounded buds, except the great 

 antennae, which are longer. The remaining limbs are as yet only 

 indicated by notches. In the greater development of the anterior 

 limbs, the animal certainly bears a resemblance to the Nauplius 

 form of Cyclops and many other Crustacea. 



At this time, too, we may discern the carapace as a flat plate, 

 covering little more than half the body (Fig. 20), somewhat 

 notched behind ; while from the centre of the notch proceeds the 

 terminal spine, no appearance as yet being visible of any approxi- 

 mation to the lateral margins, such as that which subsequently 

 converts it into a bivalve shell. The enveloping skin is now cast 

 off, as was previously the vitelline membrane, and development 

 rapidly proceeds, l^vo patches of pigment indicate the future 

 eyes ; the heart begins to beat, at first slowly, and part of the 

 alimentary duct can now be discerned ; the large oil-globule has 

 disappeared, but the smaller cells are still very conspicuous. The 

 posterior limbs are gradually formed, and the creature is ready for 

 an independent existence. Such is the course of what Sir John 

 Lubbock calls the agamic eggs. The ephippial eggs pursue a 

 course differing in many important respects, but I have not been 



