226 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



of the future caudal bristles. In front of the end of the abdominal process, two minute 

 conical prominences, at fii'st marked by similar, but fewer clear striae, gradually raise them- 

 selves up on each side fi"om the sm-face of the blastoderm and elongate, their apices being 

 directed backwards. They are the rudiments of the antennules and antennae. 



A delicate structureless membrane is now visible, covering these parts and the adjoining 

 portions of the germinal membrane. It is produced into the terminal setae of the end of 

 the abdomen and of the two pairs of appendages, and is the commencement of the first 

 skin of the larva*. 



The anterior part of the blastoderm is wider than the posterior, and is produced into 

 two great lobes divided by a median fissure. These are the " procephalic lobes," and have 

 the same relation to the anterior division of the head as the corresponding parts in the 

 embryo Aphis. 



In this state the embryo becomes a larva, for it bursts its viteDine envelope and lies 

 naked in the pouch of the mother. The rudimentary abdomen is at the same time ex- 

 tended, so that the little creature is now about ^th of an inch in length, and is very like 

 a pear in shape, the stalk being represented by the abdomen, which is terminated by a 

 flattened, bifid, spinulose fin. 



The whole larva is covered by a continuation of the delicate membrane ah-eady noticed 

 on the limbs and abdomen. The blastoderm invests the abdomen almost completely, but 

 in front it covers only a somewhat fiddle-shaped area on one face of the yelk. It is still 

 more deeply bilobed in front, and the antennules and antennae are much elongated. 

 The larva next begins to grow, being doubtless nourished by the flmd contained in the 

 maternal pouch ; and at the same time its ventral region assumes a cui've, contrary to 

 that which it originally possessed, becoming more and more convex. 



The cephalic region is now clearly distinguishable ; it occupies nearly one-half of the 

 whole length of the body. The procephalic lobes extend upwards over the anterior face 

 of the vitellus, and upon each a large rounded elevation, the rudiment of the ophthalmic 

 peduncle, has made its appearance in front of the antennule. The latter, like the antennae, 

 elongate greatly, and become divided longitudinally, within the sheath afforded by the 

 primitive integument, into their two terminal branches, 



A slight constriction indicates the boundary between the antennulary and antennary 

 sterna, and behind these, similar depressions mark off the surface of the blastoderm into 

 seventeen additional segments. 



Attached to them are as many pairs of appendages, which in the youngest larvae 

 examined had the following form : — 



The first pair were rounded massive elevations, situated one on each side of the pit 

 indicating the position of the oral aperture ; from their anterior edge a short oval palp 

 already projects. These are the rudimentary mandibles. 



The next pair, or first maxillae, are small rounded elevations meeting in the middle line. 

 The second maxillae succeed, and are more elongated, three-jointed, and bent back parallel 

 with one another. The maxillipedes and the thoracic ambulatory legs form one continuous 



• See, for illustrative figures of the development o( Mysis, my 'Lectures' above cited. 



