COPEPODA. 121 



and at a later stage yield exclusively entoderm-elenients. They are 

 distinguished as the central {en) and the anterior (vn) entoderm-cells. 

 The four blastomeres (lateral cells) situated symmetrically on each 

 side of these two, yield, by future division, both entodermal and 

 ectodermal elements. The cleavage-sphere [u), which lies behind 

 the central entoderm-cells, also appears to be of importance. This 

 divides later into four elements, two larger anterior cells representing 

 the primitive mesoderm-cells (Fig. 57 B, um), while two posterior 

 •cells become ectodermal elements. 



Fig. 57 B shows us the central entoderni-cell (en) divided to form 

 two blastomeres ; the lateral entoderm -elements (sn) have also 

 become distinct by the division of the lateral cells. We thus have 

 the rudiment of the entoderm consisting of seven cells, behind 

 which lie the two primitive mesoderm-cells {ain). The immigration 

 of the mesoderm elements towards the centre of the embryo next 

 takes place. The primitive mesoderm-cells yield by division two 

 laterally placed elements (Fig. 57 C, m, and «?n), and these four 

 mesoderm-cells (of Avhich the two median cells are to be considered 

 as the pole-cells of the lateral mesodermal band) shift into the 

 segmentation-cavity (Fig. 57 C). Soon after this there occurs 

 the invagination of the entodermal elements {en), by which the 

 gastrula-stage is reached (Fig. 57 C). The blastopore, a longitudinal 

 fissure (Fig. 57 D), now closes from before backward, and the ento- 

 derm which has sunk inwards thus becomes a closed vesicle. It 

 appears that the blastopore corresponds in position to the future 

 ventral side of the embryo. If so, the part that closes latest would 

 lie in the neighbourhood of the future anal aperture. 



The stomodaeum and the proctodaeum, according to the researches 

 of Urbanowicz in connection with Cyclops, arise as ectodermal 

 invaginations, the former appearing during embryonic development, 

 while the latter develops only in the earliest larval stage. They 

 both become connected with the archenteric vesicle. 



The gastrula-stage iu the Copepoda was tirst investigated and described hy 



HOEK. 



The statements made by Urbanowicz as to the formation of the germ-layers 

 iu Cyclops do not agree witli Gkobben's views. In Cyclops there is, at first, only 

 one entoderm-cell which sinks inwards, the blastopore closing above it; this cell 

 then yields by division the whole entodermal rudiment. A mesencliyme next 

 arises by the abstriction of ectodermal elements, this mesenchyme giving rise to 

 most of the mesodermal structures of the NaupUus, while the actual mesoderm 

 is a later, secondary structure, probably originating from the entoderm and 

 yielding exclusively the mcsoderm-band. When, however, we take into con- 



