432 FORMATION OF THE LAYERS. 



mesenteron. The stomodeeum is said to be formed at the point of closure 

 of the blastopore. The mesoblast passes inwards and forms a mass ad- 

 joining the hypoblast, and somewhat later the genital mass also becomes 

 covered by the epiblast. The proctodseum appears to be formed later than 

 the stomodsBum. 



The embryo as first shewn by Dohrn passes through a Nauplius stage 

 in the brood-pouch, but is hatched, except in the case of the winter eggs of 

 Leptodora, in a form closely resembling the adult. 



Copepoda. Amongst the free Copepoda the segmentation and forma- 

 tion of the layers have recently been investigated by Hoek (No. 512). He 

 finds that there is, in both the fresh-water and marine forms studied by him, 

 a centrolecithal segmentation similar to that of Palsemoii and Pagurus 

 (vide p. 92), which might from the surface be supposed to be 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 transverse 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 blastodermic thickening. 



Amongst the parasitic Copepoda there are found two distinct types of 

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

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

 Eupagurus, etc. (vide p. 92). In the other group (Anchorella, Clavella, Con- 

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

 meroblastic type (vide p. 99), 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 enclosed 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 em- 

 bryo develops till it reaches the so-called Cyclops stage (vide p. 407). The 

 embryo within the egg has its abdomen curved dorsal wards 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 (?). 



