212 K. E. VON BAER. PHILOSOPHICAL FRAGMENTS. 



and Isopoda, which in their perfect condition are far more differ- 

 ent from the Decapoda than these from one another. To this it 

 must be added, that, according to Rathke, the heart in the De- 

 capoda (IsiSy Bd. xvii. p. 1098) is at first fusiform, and doubtless 

 there are many more points of agreement as yet undiscovered. 



At a still earlier period, when the feet sprout forth at the sides 

 like little knobs, and no gills are as yet visible, we cannot mis- 

 take the similarity to true Insects in the larval state. A Butter- 

 fly and an Ichneumon may be readily mistaken for one another 

 even as full-grown larvae. Such larvae have indeed been com- 

 pared with Vermes, but it must be confessed that the essential 

 differences are still very great. The latter have red blood and 

 no tracheae. In the former the reverse obtains. In fact, the 

 full-grown caterpillars are much more similar to the Myriapoda, 

 and only at a very early period, when no tracheae are developed 

 (these being probably formed by histological differentiation), is 

 there any approximation to the embryo of the Leech when it 

 has not yet developed red blood. 



These remarks lead us to inquire, whether by going further 

 and further back, we may not eventually attain a stage in which 

 the embryos of the Vertebrata agree with those of the Inverte- 

 brata. In a future essay, in which the differences in the schemes 

 of development in the principal types of animals will be treated 

 of, I shall endeavour to show that the Articulate series also com- 

 mences its development by a primitive streak. For this short 

 period, therefore, there would be an agreement between them 

 and the Vertebrata. In the condition of the actual germ, how- 

 ever, it is probable that all embryos which are developed from 

 true ova agree. This is a strong reason for considering the germ 

 as the animal itself (Schol. II.). When in the germ of the Bird 

 the primitive streak is developed, we are indeed inclined to say, 

 Now commences the embryo. But in reality this is only the 

 instant in which the vertebrate type appears in the germ ; for the 

 primitive streak is nowise the whole embryo, inasmuch as the 

 parts which are metamorphosed into the abdominal plates evi- 

 dently lie beside it in the germ. It is only that part of the 

 germ which first becomes individualized. A so-called germinal 

 disc is however distinctly visible in the ova of the Articulata. It 

 is almost certainly present in the Mollusca, for the ovum of 



