vm ARTHROPOD A 171 



colonial Protozoa are found, however, when closely examined, to 

 exhibit similar strings of cytoplasm connecting together the various 

 individuals which make up the colony, and so the opposition between 

 the two views tends to disappear. 



The mass of scattered endoderrn cells undergoes contraction, its 

 units being drawn closely together, so that it forms a compact group 

 of cells, and then the ectoderm grows over its sides and completely 

 invests it, leaving only a small area in the centre of the vegetative 

 pole uncovered. At this spot the large rounded endoderm spheres 

 protrude for a time (Fig. 123 C). Soon a cavity is formed in the 

 centre of the endodermic mass, by the formation of vacuoles which 

 coalesce with one another. This cavity, which is the archenteron, 

 opens to the exterior by an aperture, the blastopore, in the centre 

 of the uncovered area of endoderm, and so the process of gastrulation 

 is completed. 



The gastrula, which had become nearly spherical, now again 

 elongates, and the blastopore becomes elongated also. Behind it there 

 appears a darker area which seems to be an area of rapid proliferation 

 in the endoderm, this is named by Sedgwick the primitive streak. 

 From this area there is produced a crescentic mass of cells lying 

 beneath the endoderm, the two horns of which grow forwards at the 

 sides of the blastopore and constitute the two mesodermic bands. 



In the meantime the elongated blastopore becomes divided by a 

 constriction into two apertures, the anterior of which persists as the 

 mouth whilst the posterior remains as the anus. The mesodermic 

 bands then became divided into blocks termed somites, in each of 

 which a cavity, the coelomic cavity, appears (Fig. 124). 



For some time the blastopore is considerably less in length than 

 the embryo, so that there is a prae-oral as well as a post-anal gut, or 

 to put it in another way, there is a short ventral surface and a very 

 long arched dorsal one. The prae-oral and post-anal gut finally 

 disappear owing to the greater relative growth of the ventral surface. 



The reader will not fail to observe that up to this stage there 

 is a remarkable general resemblance between the development of 

 Peripatus and that of an annelid. The formation of a cap of small 

 ectoderm cells resting on larger endoderm cells and gradually 

 investing the latter by the process termed epibole ; the division of 

 the blastopore into mouth and anus ; the. formation of mesodermic 

 bands from endoderm cells in the posterior lip of the blastopore, and 

 their division into metamerically 'arranged somites, in each of which 

 a cavity appears ; all these are features which have become famili;ir 

 to us in our study of the development of Annelida. 



But from this stage onwards distinctively arthropodan features 

 make their appearance. The rudiments of appendages appear as pairs 

 of protrusions of the ventral ectoderm arranged metamerically behind 

 one another in correspondence with the somites ; the first to appear 

 are the antennae which are at first situated at the sides of the mouth, 

 but which later, along with the corresponding somites, shift forwards 



