402 DESIGN IN NATURE 



left to chance or even to the individual effort of any particular organism either during its developmental or adult 

 condition. In the case of the frog the embryo begins its career as a fish does. It has a ventral bag of pabulum 

 (vitellus), no alimentary canal or mouth, no tail, and no gills (branchiae). As development proceeds the bag of 

 pabulum disappears and a mouth and alimentary canal are formed, a swimming tail, and gills or water -breathing 

 outside lungs make their appearance. Further on, the tail shrinks and falls off, and four rudimentary hmbs are 

 provided instead. The gills or outside lungs adapted for water-breathing also give place to inside or true lungs 

 adapted for air-breathing. These changes, be it remarked, are not due either to the disuse or the inadequacy of the 

 tail and gills, but only to the exigencies of design and final purpose. They are means to ends, and the means are 

 employed to secure the ends in anticipation, and according to pre-arrangement. In the menobranchus (a fish-like 

 animal) the gills are persistent, and so are the limbs (Plate xciii., Fig. 4, page 40.3). 



PLATE XCIII 

 Plate xciii. illustrates development and peculiarities in the fish, frog, menobranchus, and other animals. 



Fifi. 1.— Egg of fish. 



A. Egg of fish, showing formation of umbilical vesicle. 



B. Young fish with umbilical vesicle. 



C. Human embryo, with umbilical vesicle (about the fifth week). 



Fig. 2. — Shows the development of the frog. 



A. Diagram of frog's egg, in an early stage of development, as seen in longitudinal section, o, Thickened portion of external 

 blastodermic layer, forming body of fo?tus ; h, anterior extremity of fretus ; c, posterior extremity ; d, internal layer of blastodermic 

 membrane ; e, cavity of vitellus. 



B. Egg of frog, in process of development ; tail appearing. 



C. Egg of frog, further advanced ; eye appearing, tail increasing in size. 



D. Tadpole fully developed ; eye more advanced, and mouth and alimentary canal formed. 



Fig. .3. — Shows tadpole developing limbs and adult frog. In the first figure the tail is diminished in volume, and rudimentary 

 limbs are appearing. The gills or branohife are also in a transition stage ; in other words they are changing from outside water- 

 breathing lungs to inside air-breathing ones. In the second figure the frog is fully developed. It has lost its tail and is provided 

 with four powerful limbs and true air-breathing lungs. 



Pig. 4. — Head and gills of Menobranchu.t. In this curious creature the outside branchiae or water-breathing lungs and the 

 limbs are persistent. 



Fig. 5. — Gills of a, ganoid fish (C'eratodus). Affords a good example of the outside or water-breathing lung. ((, Arcus aortse ; 

 /), gills containing rows of capillary blood-vessels ; c, pseudo-branchia ; d, d', two series of gill-rakers belonging to the pseudo- 

 branchia ; e, ceratohyal ; /, glossohyal (after Gunther). 



Fig. 6. — Diagram of the larva of a calcareous sponge in its gastrula phase. The larva at this stage consists of a simple pouch 

 or stomach with a mouth (a). The walls of the stomach are formed of two layers of cells, an internal and an external ; the external 

 cells being elongated and furnished with filaments (cilia), which serve as organs of locomotion. These two layers of cells have been 

 likened to the internal and external layers of the blastodermic membranes in the higher animals (after Haeckel). 



Fig. 7. — Ascidian. 



A. Early tailed larva. 



B. Tailed larva, seen on the right side (altered from Seeliger). 



0. Transverse section through the tail of an ascidian larva (after Seeliger). 



A. o. Brain ; h, neural tube ; c, notochord ; d, mesenteron. 



B. a, Brain, with eye and ear ; h, nerve cord ; c, notochord ; d, atriopore ; e, intestine ; /, remains of caudal intestine ; g, neuro- 

 pore ; h, papilla ; i, laryngeal clefts ; k, endostyle in wall of pharynx ; j, heart. 



C. a, Median fin ; /), nerve tube ; c, notochord ; d, muscle ; e, caudal hypoblast. The parts here enumerated foreshadow 

 advancing types ; a vertebral column, nervous system, alimentary canal, vascular system, and sense organs being all indicated. 

 The ascidian is to be regarded as intermediate between the molluscoid invertebrates and the vertebrata. 



Fig. 8.-0, h, r, d, Polar capsules of My:a)bohis ellipsoiden (after Balbiani). Illustrates spiral, vibratile swimming arrangements 

 in rudimentary structures. 



Fig. 9.— Shows a series of embryos exhibiting similarity of structure and development in their primary and secondary stages. 

 The upper row shows the gill arches and the absence of limbs ; the lower row shows the partial disappearance of the gill arches and 

 the budding of the limbs. 



A. Gill arches and rudimentary limbs in tortoise. 



B. Gill arches and rudimentary limbs in fowl. 



0. Gill arches and rudimentary limbs in echidna. 



D. Gill arches and rudimentary limbs in koala or Australian bear. 



E. Gill arches and rudimentary limbs in rabbit. 



F. Gill arches and rudimentary limbs in pig. 



G. Gill arches and rudimentary limbs in deer. 

 H. Gill arches and rudimentary limbs in cat. 



1. Gill arches and rudimentary limbs in monkey. 



J. Gill arches and rudimentary limbs in man (after Haeckel). 

 Note.— -T\ie figures A to J of Fig. 9, representing stages of development in various animals, and consisting as they do of mere 

 repetitions of parts, indicate a general plan or type. They afl:ord no proof of evolution, or the production of the higher animal 

 forms from the lower ones. The upper and lower rows of figures, taken separately, show repetition as apart from advance. 



