578 WILLIAM PATTEN. 



each extremity is continuous with the amnion (PI. XXXVIa, figs. 

 9 and 10, am.^ and am."). After the appearance of the procto- 

 dseum the posterior end of the body becomes bent forward on 

 itself, making the first step towards the revolution of the embryo. 



Formation of the Appendages. — Shortly after the growth 

 of the ventral plate has brought the two extremities into close 

 proximity with each other, six pairs of gentle undulations 

 appear simultaneously on the anterior half of the body (PI. 

 XXXVIa, fig. 8) . The three posterior pairs, which ultimately 

 become the thoracic appendages, are somewhat longer than the 

 others. The three anterior pairs become respectively the man- 

 dibles, and the first and second maxillse. As development 

 proceeds each of these appendages assumes a characteristic 

 shape and direction of growth. The mandibles (PI. XXXVIa, 

 fig. 9) become globose, and lie at right angles to the median 

 plane of the body. The first m axillae diverge very early from 

 their original rounded form, becoming elongated and pointed at 

 their extremities ; they are much larger than either the man- 

 dibles or the second maxillae, and maintain their superiority in 

 size until the embryo hatches. The second maxillse imitate the 

 first maxillse in shape and direction of growth. 



The three thoracic appendages are very similar to each other 

 in size and outline and in their manner of development. When 

 they are about half grown three pairs of rudimentary appen- 

 dages may be seen in each of the three anterior abdominal seg- 

 ments (PI. XXXVIa, fig. 11). The thoracic appendages are at 

 first directed towards the median line of the germinal band, 

 being in marked contrast with the rudimentary jaws, which are 

 directed away from the middle line of the germinal band. No 

 further important changes occur either in the direction of 

 growth or in the shape of any of the appendages until just 

 before revolution, when the second maxillse fuse to form the 

 under lip and the first maxillse at the same time turn towards 

 the median ventral line and assume their ultimate arrange- 

 ment around the mouth- opening. The thoracic appendages 

 become very much distorted just before the rupture of the 

 amnion and serosa (PI. XXXVIa, fig. 14). 



