520 FORMATION OF THE LAYERS. 



complete and nearly regular. After the formation of the blastoderm an 

 invagination of some of its cells takes place and is completed in about a 

 quarter of an hour. The opening becomes closed. This invagination is 

 compared by Hoek to the invagination in Astacus, and is believed by him 

 to give rise to the mesenteron. Its point of closing corresponds with the 

 hind end of the embryo. On the ventral surface there appear two trans- 

 verse furrows dividing the embryo into three segments, and a median 

 longitudinal furrow which does not extend to the front end of the foremost 

 segment. The three pairs of Nauplius appendages and upper lip become 

 subsequently formed as outgrowths from the sides of the ventral blasto- 

 dermic thickening. 



Amongst the parasitic Copepoda there are found two distinct types of 

 segmentation, analogous to those in the Isopoda. In the case of Condra- 

 canthus the segmentation is somewhat irregular, but on the type of Eupa- 

 gurus, etc. (vide p. 112). In the other group (Anchorella, Clavella, Congeri- 

 cola, Caligus, Lerneopoda) the segmentation nearly resembles the ordinary 

 meroblastic type (vide p. 120), and is to be explained in the same manner as 

 in the cases of Oniscus and Cymothoa. The first blastodermic cells some- 

 times appear in a position corresponding with the head end of the embryo 

 (Anchorella), at other times at the hind end (Clavella), and sometimes in the 

 middle of the ventral surface. The dorsal surface of the yolk is always 

 the latest to be inclosed by the blastoderm cells. A larval cuticle similar 

 to that of the Isopoda is formed at the same time as the blastoderm. At 

 the sides of the ventral thickening of the blastoderm there grow out the 

 Nauplius appendages, of which only the first two appear in Anchorella. 

 In Anchorella and Lerneopoda the embryos are not hatched at the 

 Nauplius stage, but after the Nauplius appendages have been formed 

 a fresh cuticle the Nauplius cuticle is shed, and within it the embryo 

 develops till it reaches the so-called Cyclops stage (vide p. 490). The 

 embryo within the egg has its abdomen curved dorsalwards as amongst the 

 Isopoda. 



Cirripedia. The segmentation of Balanus and Lepas commences by 

 the segregation of the constituents of the egg into a more protoplasmic 

 portion, and a portion formed mainly of food material. The former sepa- 

 rates from the latter as a distinct segment, and then divides into two not 

 quite equal portions. The division of the protoplasmic part of the embryo 

 continues, and the resulting segments grow round the single yolk segment. 

 The point where they finally enclose it is situated on the ventral surface 

 (Lang) at about the position of the mouth (?). 



After being enclosed by the protoplasmic cells the yolk divides, and gives 

 rise to a number of cells, which probably supply the material for the walls of 

 the mesenteron. The external layer of protoplasm forms the so-called 

 blastoderm, and soon (Arnold, Lang) becomes thickened on the dorsal 

 surface. 



The embryo is next divided by two constrictions into three segments ; 

 and there are formed the three appendages corresponding to these, which are 



