6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



must keep one surface against the support, and thus it estabHshes a 

 functional distinction between its upper surface and its lower surface, 

 which has led to the structural differentiation of dorsivcntrality ; and 

 from this, in combination with movement in one direction, finally, 

 bilateral symmetry of organization necessarily follows. 



Progression by crawling instead of by swimming alters the whole 

 status of the relation between the animal and the environment. A 

 mouth at the posterior end of the body now becomes quite impracti- 

 cable, and embryonic history shows that crawling animals proceeded to 

 rectify the defect, supposedly inherited from their free-swimming 

 ciliate ancestors, by lengthening the mouth, or blastopore, in a for- 

 ward direction on the under side of the body. In the young Peripatus 

 embryo, for example (fig. 4 A, B), the l)lastopore is a long slit on the 



Fig. 4. — Early stages in the development of Peripatus capensis. (From Balfour, 

 1883.) 



The blastopore {Bp) elongates on ventral surface of embryo, and then 

 closes except at the two ends (C) where open extremities become mouth and 

 anus. Segmentation appears as series of coelomic sacs in mesoderm (see fig. 6, 

 Msd). 



ventral surface of the blastoderm. Later, the edges of the slit come 

 together (C) and unite except at the two ends, where openings re- 

 main into the archenteron that become the mouth and anus of the 

 mature animal. In insects and other arthropods, the process of gas- 

 trulation in the embryo (fig. 5 A) is clearly a modification of that in 

 Peripatus, by which many of the details have been omitted and the 

 whole procedure greatly altered. In most insects (fig. 5B), gastru- 

 lation resembles that of the planula (fig. 2) in so far as it takes place 

 by an internal proliferation of cells from the blastoderm, but most 

 of the gastrulation area gives rise to mesoderm, the true endoderm 

 being formed only at the two extremities of the inner layer (fig. 5 C, 

 AMR). 



