Dr. W. Salensky on HackeVs Gastraia Tlieory. 19 



in a manner differing from the true Planula-ioxm^ but yet re- 

 main perfectly homologous with the germ-lamellaj of the 

 Planula. These processes also appear to take place in the 

 same way in the scorpion. 



Cases may, however, occur in which, after segmentation, a 

 Plamda-ioxm. is not at once produced. Most of these cases 

 have been recently made known by the researches of Kowa- 

 levsky and Mecznikoff in the Ascidia, Amphioxus^ Nemertina, 

 &c. In these animals the c^^g passes through a so-called re- 

 gular segmentation, and at the close of this becomes converted 

 into a vesicle surrounded by uniform cells, which, to distinguish 

 it from the Planula, may be named the " Dlastulay The di- 

 stinctions between the Planula and the Blastula are that the 

 former already possesses two germ-lamella?, while the latter 

 has still to form them. As the Planula-form. in the Coelen- 

 terata issues from the egg and passes into free life, so also can 

 the Blastula become free and swim about in the water, as is 

 the case, for example^ in the Nemertina (Mecznikoff, ' M^moires 

 de I'Acad. Imp. de St. Pctersb.' tome xiii.). In such a larval 

 or developmental stage we can say nothing of either exoderra 

 or entoderm. The two lamella are still quite undifferentiated; 

 this differentiation occurs somewhat later, and leads to a form 

 which differs somewhat from the Planula-iorva.. In some 

 cases, before the differentiation into two germ-lamellae, this 

 Blastula-iomx may form a thickening at one point of its surface, 

 to which the subsequent differentiation is confined, as seems to 

 be the case, for example, in the Mammalia. Usually the dif- 

 ferentiation commences in the Blastula by a portion of its cells 

 beginning to distinguish themselves from the rest by some 

 character. 



Let us commence our examination with the processes which 

 indicate the differentiation in the Blastula of the Ascidia, as 

 these have been best investigated. The first alteration in the 

 Blastula consists in its becoming flattened on one side *. From 

 Kowalevsky's figures we see that at this stage (see Kowalevsky 

 loc. ci't. fig. 5 and PI. V. fig. 4) the two germ-lamellaj are 

 already differentiated. The differentiation occurs in the same 

 way in LumhricuSy where also the same flattening of the Blas- 

 tula is the first thing that makes its appearance. I must spe- 

 cially cite this first form of the differentiation of the germ- 

 lamelljE, because in most cases in the above-mentioned animals 

 the differentiation of the gerai-lamellge has been confounded 

 with the subsequent invagination ; the latter, however, is a 



* Kowalevsky, " Weitere Studien iiber die Entwickelung der einfacben 

 Ascidien," in Archiv fiir mikr. Anat. Bd. vii. p. 105. 



2* 



