412 GENERAL CONSIDERATIONS ON 



Superficial cleavage, as a rule, occurs only in the Arthropoda. 

 Where other forms, e.g., Renilla, Glavularia (Vol. i., p. 76) show a 

 similar method in the first stages, this does not lead to the same 

 results as typical superficial cleavage, viz., to a unilaminar blastoderm 

 covering the whole surface of the egg with a uniform layer and an 

 accumulation of food-yolk filling the cleavage-cavity.* 



The formation of the germ-layers is introduced by gastrulation, 

 which, in many cases, is of the invagination-type (Moina, Lucifer, 

 Astacus, Peripatus, Hydropliilus), in others, on the contrary, gastru- 

 lation is replaced by a solid ingrowth of cells (Ligia, Limulus, 

 Scorpiones, Araneae, Myriopoda). The position of the blastopore 

 varies in the different groups. As a rule, the blastopore corresponds 

 to the ventral side of the body. 



In Peripatus and the Insecta, the blastopore is an exceedingly long 

 slit, the anterior end of which corresponds in position to the mouth, 

 and the posterior end to the anus (Figs. 99, 134, and 145). In the 

 Crustacea, on the contrary, the blastopore is said to belong to the 

 posterior end of the germ-band, and to correspond more or less in 

 position with the later anal aperture. The accounts given of the 

 Arachnida seem to indicate that, in position, the blastopore may 

 be related to the anus. 



The act of gastrulation leads to the breaking up of the common 

 rudiment of the entoderm and the mesoderm. The rudiment of the 

 mesoderm in the Arthropoda is always multicellular, except perhaps 

 in a few quite isolated cases, such as Cetocliilus. In the Insecta, the 

 formation of the mesoderm may be traced back to a folding of the 

 lateral diverticula of the archenteron (Figs. 154 and 155, p. 314). 

 The processes that take place in Peripatus may perhaps be interpreted 

 in the same way, although in this form we are inclined to assume, 

 in agreement with the Annelida, the development of two mesoderm- 

 bands advancing from behind forward through the multiplication of 

 cells. The facts as yet known of Peripatus seem rather to support 

 this last view. The question whether the condition found in the 

 Insecta (i.e., the rise of the mesoderm from the archenteron through 

 folding) represents a primitive or a derived condition, is connected 

 with the as yet unsolved problem of the first (phylogenetic) rise of 

 the mesoderm. 



In the Crustacea, the mesoderm arises in the form of a growth at 

 the lips of the blastopore. The same is most probably the case in 



* [For a comparison of the cleavage and formation of the germ-layers in the 

 Arthropoda, see Wagner (No. X.). — Ed.] 



