EMBRYOLOGY OP CLEPSING, 29.^ 



is at present wanting, but the facts seem to point in this 

 direction very strongly. The explanation of the cleavage-cavity, 

 however, does not depend upon the decision of this question, but 

 upon the fact that ceils lengthen and push each other apart in 

 cleaving. This cavity arises very early in Clepsine, and at the 

 place where the first three plains of division cross one another 

 (fig. 79, seg. c). According to Gotte (-pry)? it forms in precisely 

 the same manner in the amphibian egg. Here, as in Petromyzon 

 (-H-.})j the wall of the Blastula is at first one cell thick, but soon 

 becomes several cells thick. A transitory cavity appears in the 

 eight-cell stage, in the egg of osseous fishes, according to His 

 (V)- The same was observed by Bambeke (9) and Lere- 

 bouUet (102). Balfour and SchuUz did not find it. Oellacher 

 (124') doubts the existence of such a cavity, but has certainly 

 indicated something of that nature in his fig. 2-1, PI. XXXIII. 

 Kowalevsky, Owsjaunikow, and X'. Wagner (-rrV) testify to the 

 occurrence of the same in an early stage (6 — 8 segments) in 

 the egg of the Sturgeon. Eauber (— ip-) alleges that a segmen- 

 tation cavity, entirely distinct from the later embryonic cavity 

 ("Keimhohle "), occurs in the egg of the bird at a time when 

 only four segments have been formed. According to Kowalevsky 

 (V), a small hole appears between the first four blastomeres in 

 Ascidia, which increases in size during the cleavage, and becomes 

 the blastocoel. The Blastula form is reached in the seven-cell 

 stage in Pedicellina (Hatschek, -/vt); ^^^^ ^^ about the same 

 time in Holothuria (Selenka, tt-,t)- 



In Geryouia (Fol. ^V'V) it arises between the 8-cell and 32-cell 

 stages. Mitschnikoff (-rnMnr) says that no blastocoel occurs in 

 certain Aeginidse. Rabl (-fvv)^ ^^'l^o seems to be very successful 

 in confirming some of HackePs most doubtful views {e. g. origin 

 of mesoderm, moneriila, and morula), asserts that in fresh water 

 Pulmonata (Lymnseus, &c.) the cleavage ends in a morula, in 

 the centre of which a blastoccel subsequently forms. In Euaxes 

 (Kowalevsky) the blastocrel appears in the 8-cell stage, just as 

 ill Clepsine. In Lumbricus (~) it is never much more than a 

 simj)le fissure. In CucuUanus elegans (Biitschli, -Vr) ^.he 

 cleavage ends in producing two cell-plates, between which there 

 is no open space. Only a narrow fissure is found between the 

 cleavage elements in Paludina C^-,^). 



P. E. Schultz (t^I) describes an interesting segmentation- 

 hole in the egg of Sycandra, The cleavage cells, sixteen in 

 number, form two rings of eight each (apical and basal). The 

 cleavage hole passes through the centre of each ring, and is a 

 little smaller in the apical than in the basal ring. Both ends of 

 the hole finally close, but the apical first. Thus a simple cavity 

 is formed. According to Plemming (tVt)^ a lenticular cavity 



VOL. XVUI.- — NEW SER. U 



