286 



EMBRYOLOGY 



Both the origin and the further development of the mesoderm are dis- 

 puted points in the development. In some cases, as, for example, in the 

 Lumhricus studied by Kowalevsky (and in Nereis, according to Goette), 

 it appears as if the first mesoderm cells had been derived from the ento- 

 derm cells, whereas in other cases they seem rather to have belonged to 

 the ectoderm. Usually their origin cannot be referred to either one or 

 the other of the two primary germ-layers, for they were established before 

 the formation of these, or on the border-line of the two. Such is the 

 case in various OligochiBta and also in Polychreta. In EJiynchelmis 

 (comp. p. 282) the so-called mesomeres are separated off from the 

 large blastomeres, which subsequently become the entoderm, and, to- 

 gether with the micromeres, overlie these ; apparently therefore they 

 belong to the ectoderm. It is only later that they are overgrown by the 



ectoderm, and move to the inside 

 here to develop into the mesoder- 

 mal bands (Vejdovsky). 



Just as the opinions of authors 

 are divided in regard to the deri- 

 vation of the mesoderm, so also 

 are they in regard to the manner 

 in which the mesodermal bands are 

 formed. Whereas some authors 

 derive them from proliferations of 

 the primitive mesoderm cells (Kow- 

 alevsky, No. 27; Hatschek, No. 

 18 ; Goette, No. 15), others are of 

 the opinion that the parts of the 

 ectoderm which lie over the meso- 

 dermal bands also supply cells for 

 the I'einforcement of these bands, 

 and that as a result ectoderm and 

 mesoderm are in this region con- 

 tinuous (Fig. 129 B). Kleinenbero 

 (No. 24) thus describes the condi- 

 tions in Lvmbricus trapezoides. Salensky (No. B7) agrees with him. 

 Kecently Kleinenberg (No. 26) has gone still further, for he considers 

 that the entire mesoderm— the existence of which as a separate layer he, 

 moreover, denies— has been gradually split off from the ectoderm. This 

 point will be referred to again in considering the organogeny. 



Fig. 132.— An embryo of Lumhricus 

 agricola, already far advanced in deve- 

 lopment (after Kowalbvskt). h, pos- 

 terior part of the embryo, resembling 

 a yolk-sac ; its wall is formed of ecto- 

 derm and entoderm, and it gradually 

 becomes overgrown by the mesodermal 

 bands ; mes, upper limit of the left 

 mesodermal band ; as, oesophagus. 



3. The Formation of the Organs. 



So much of the formation of the individual organs as has 

 not been considered in the two preceding sections upon the 

 general form of the body will be added here. However, it 



