DESCRIPTION OF PLATE Yll-coniinued. 



Eig. 10. — Tlirough a simple Acalepha (Hydroid). Between the gastral 

 layer («) and the neuro-dermal layer (k) lies the fibro-dermal 

 layer. 



Fig. 11. — Through an Acoelomatous embryo (Turbellaria). The section 

 goes right through the primitive brain or oesophageal 

 ganglion (n). Between the neuro-dermal layers (h) and the 

 intestinal glandular layers (d) are visible, moreover, the two 

 fibrous lamellse which lie compactly on one another — the 

 outer the fibro-dermal layer {m), and the inner the fibro- 

 intestinal layer {/). 



Fig. 12. — Through an Ascidian larva, from the base of the tail, so as to 

 put in the lowest end of the chorda (.t) between the medullary 

 canal (n) and the intestinal tube {(I). Between the fibro- 

 dermal layer (m) and the fibro-intestinal layer (/) the ccelom 

 is visible. 



Eig. 13. — Through an Amphioxus larva (compare Kowalevsky's ' De- 

 velopment of Amphioxus' plate ii, fig. 20). The fibro- 

 intestinal layer (f) is still entirely separated from the 

 fibro-dermal layer {m) ; the entire body becomes barely 

 formed from the four secondary germ-lamellae. 



Eig. li. — Through an older Amphioxus larva. Tlie medullary canal {n) 

 has become completely unravelled out of the horny layer {h). 

 The fibro-dermal layer {m) is blended together with the fibro- 

 intestinal layer (/) in the dorsal middle line (mesenteric 

 line), and is differentiated into the skin (/) and hollow 

 muscles (r). Between the intestinal tube and the unravelled- 

 out medullary canal (n) is seen the commencement of the 

 chorda (x). 



Fig. 1-5. — Through a worm-embryo (cephalic segment of an Annelid). 

 Between the dorsal (r) and ventral {b) longitudinal muscles 

 are interposed the primitive kidneys (segmental organs, u) 

 from the outer dermal layer throughout the body-cavity (c). 

 On the upper side of the primitive intestine {a) is the dorsal 

 longitudinal vessel {t), below the same the ventral longi- 

 tudinal vessel appears {z), both enclosed in the fibro-intestinal 

 layer (/). 



Eig. 16. — Through a vertebrate embryo (central section of a fish) 

 Between the dorsal (r) and ventral {b) hollow lateral muscles 

 is interposed the first appearance of the primitive kidney 

 system {u) from the skin tiiroughout the body-cavity. On the 

 upper surface of the primitive intestine {a) is the primordial 

 aorta (f), below the same the outline of the heart (or the 

 bulbus arteriosus, z) is to be seen, both enclosed in the fibro- 

 intestinal layer (f). The only real difference between the 

 typical transverse section of the vertebrate body and that of 

 the worm-body (fig. 15) is that in the former, between the 

 medullary tube {n) and the primitive intestine («), the chorda 

 {x) makes its appearance. 



Jena ; 29th September, 1873. 



