2l6 AN INTRODUCTION TO ZOOLOGY 



as the primitive streak. Five other thickenings also arise in this 

 region, a pair of optic lobes (Fig. 113, K) a pair of thoraco-abdominal 

 plates (TA.), and a median entodermal plate (ES.). The side 

 on which these thickenings occur will become the ventral surface 

 of the adult. 



A later stage (Fig. 114) shows a median outgrowth just back 

 of the optic lobes which will become the upper lip, orlabrum (/), 

 and pairs of rudimentary antennules (a,), antenna (a 2 ) and man- 

 dibles (m). Between the antennae is a depression, which will 

 become the mouth. An opening in the thoraco-abdominal plate 

 (TA.) represents the future anus (A). In a more advanced 

 embryo (Fig. 115) the structures already mentioned are seen in a 

 later stage of development. The thoraco-abdominal rudiment 

 has become divided into a number of segments, from each of 

 which a pair of appendages arise and develop into those of the 

 adult crayfish (171). The length of time required for the various 

 stages in development was found by Andrews to be as follows: 

 " Cleavage took up the first week, the beginning of an embryo the 

 second week, to progress as far as the Nauplius the third week 

 and more, to enlarge the embryo over one half of the egg a fourth 

 week and more, and to perfect the embryo for hatching a fifth and 

 sixth week or more. The whole egg development required from 

 five to eight weeks in different sets of eggs under different tempera- 

 ture " (159, pp. 189-190). 



HATCHING (159, 163). In hatching, the egg capsule splits and 

 the larva emerges head foremost. The helpless young crayfish 

 would drop away from the mother at once but for a thread extend- 

 ing from its telson to the inner surface of the egg capsule (Fig. 

 1x6). 



Soon the larva possesses strength enough to grasp the egg string 

 with its claws (Fig. 116). The telson thread then breaks. After 

 about forty-eight hours the larva passes into a second stage. This 

 is inaugurated by the shedding of the first larval cuticular cover- 

 ing, a process known as molting or ecdysis. This casting off of 

 the covering of the body is not peculiar to the young, but occurs 



