106 



KMRRYOLOGY 



the lumen of the arehenteron can be recognized only as a 

 linear fissure in the pluf^-like ingrowth, another method of 

 formation of the lower germ-layer, that may be called polar 

 ing-ression, has been maintained by Goette. According to 

 GoETTE, the cells of the blastula have not the same form in 

 the entire circumference, but are somewhat shorter and 

 broader in one hemisphere. From this region there is a 

 migration of individual cells into the blastoccele, until finally 

 this cavity is completely filled with a solid cell-mass 

 (entoderm). The arehenteron arises in this in the form of a 

 fissure, which soon breaks through to the exterior at the 

 region from which the immigration of entoderm cells took 

 place, thereby forming the primitive mouth (prostoma). 



Even during this process the embryo, originally spherical, 

 elongates, so that the longitudinal axis passes through the 

 primitive mouth and the apical pole lying opposite to it. 

 But the primitive mouth very soon closes completely. At 

 the same time the larva becomes narrowed at this end, so 

 that it is pyriforni. The swarming out of the ciliated 

 embryo (planula. Fig. 52 A) now takes place ; the broader 

 apical pole is directed forwards in swimming, whereas the 

 narrower pole, at which the closure of the primitive mouth 

 took place, comes to lie behind. Nettling capsules very soon 

 make their appearance on the swarming larva ; these arise 

 in great numbers at the posterior pole, whereas they are 

 almost wanting at the anterior end. 



Even during the swarming stage a shallow depression is 

 developed at the anterior (apical) pole of the larva, and at 

 this point the epithelium acquires a glandular nature. The 

 larva now attaches itself by the apical pole to some support, 



duced by the formation of a distinct invagination gastrula. A migration 

 of cells into the blastoccele, as described by Goette, was also occasionally 

 observed in A. Havidula. However, these cells appear to disintegrate 

 without taking any part in the formation of the entoderm. 



On the other hand, the entoderm is formed in Cyanea arctica, according 

 to McMuiiKicH (Appendix to Litciature on Scyphomedusie, No. V., p. 814, 

 and No. VI., p. 'JO), by an inward migration of certain cells of the blasto- 

 sphere, and in Cyanea capillata, according to Hamann (No. IV., Appendix 

 to Literature), by the ingrowth from one pole of the embryo of a solid rod, 

 which subsequently becomes hollowed out to form the gastral cavity. — H.J 



