MUSEUM OF COMPARATIVE ZOOLOGY. 307 



teen, thirty-two segments, and so on, until the whole surface of the egg 

 is made of minute, polygonal cells (perimorula). No polar globules are 

 emitted.* 



Twenty-four hours after the cleavage began, an egg was found in the 

 gastrula-stage, as represented on Plate I. Fig. 3. The cavity is very 

 small compared with the size of the egg, and its orifice (g) is bounded 

 by elliptical cells, which form a light-colored disk on the surface of the 

 egg. The orifice apparently elosed a little later, and the further history 

 of the cavity was not obtained. 



The yelk now shrinks away from its membrane in the vicinity of the 

 light-colored disk, and becomes transparent and free from granules 

 preparatory to the appearance of the parts of the embryo. 



The parts of the embryo which first appear are the abdomen, the 

 labrum and cephalic disks, and the first three pairs of appendages. 

 These appear almost simultaneously. A protuberance from the surface 

 of the yelk, near the place whei-e the gastrula opening appeared, is the 

 rudiment of the abdomen. Another fold over against the first is the 

 beginning of the formation of the labrum, and in front of this two disks 

 appear, which are the first trace of the eyes and front part of the head 

 of the embryo. 



The projection of the two folds which form the abdomen and labrum 

 is such that the free ends are toward one another. Thus the abdomen, 

 from the time of its earliest appearance, is bent forward underneath the 

 breast of the future prawn. On each side of the labrum and abdomen 

 are three lobes, which are the rudiments of the two pairs of antennae and 

 the mandibles. The first pair are on a level with the labrum, the third 

 pair with the end of the abdomen. All three pairs are simple. This is 

 the nauplius-stage of the embryo, f 



* P. Mayer observed occasionally in fresh-laid eggs of Eupagurus "a sort of Rich- 

 hingsbldschen," but he considers these cases abnormal. They were probably the 

 result of disintegration of the egg (op. cit., p. 223). 



t Joly's account of the order of appearance of the appendages of the embryo of 

 Caridina, — first, the three jiairs of thoracic legs (i. e. the maxillipeds), then the 

 maxillae, mandibles, and antennaa, — is without doubt incorrect (op. cit., pp. 59, 60). 

 In an abstract of a paper " On the Development of the Crustacean Embryo, and the 

 Variations of Form exhibited in the Larva? of 38 Genera of Podophthalmia " (Proc. 

 Royal Soc. London, Vol. XXIV. p. 378. 1876), Spence Bate states his belief "that 

 he has demonstrated that the three pairs of mobile appendages in the cirripedal or 

 NaiqMus form of larva homologize with the eyes and two pairs of antennae, and not 

 with the antennae and mandibles, as stated by Fritz Miiller, Anton Dohru, and 

 others." (!) It is to be hoped that the evidence will not long be withheld from 

 publication. 



