344 SPARKS FROM A GEOLOGIST'S HAMMER. 



derivatively related to each other. Trace any higher ver- 

 tebrate man himself, if you will from a primitive 

 condition in the ovum. How marvelous, how awe-inspir- 

 ing is the unfolding! We have first the yolk, with its 

 " germinative vesicle " and " germinative dot. 1 ' Then both 

 undergo a succession of segmentations until there results 

 a crowded mass of cells (' morula" or "mulberry 1 ' stage). 

 Some of these dissolve, and the remainder arrange them- 

 selves as a hollow spheroid consisting of a single layer 

 of cells ("planula" stage). The single layer becomes 

 double, with an opening at one pole of the spheroid (" gas- 

 trula " stage) ; and now appears a thickening on one side, 

 in the midst of which is disclosed the " primitive furrow," 

 afterward to be inclosed and become the spinal marrow. 

 An enlargement is seen at one extremity; this is the 

 forming brain; and the various segments of the brain 

 appear as gentle swellings. At the opposite extremity 

 is a tail. Transverse marks in the middle of the neural 

 furrow indicate the approaching vertebral structures; while 

 certain segments along the place of the neck are seen to 

 receive blood-vessels from the provisional heart, and to 

 sustain completely all the structural relations of the 

 branchial or gill arches in the type of fishes. Arms and 

 legs bud out, as yet without digits, or they may be 

 viewed as unidigitate, like the limbs of Lepidosiren. 

 Stumpy digits afterward appear, like those of the so- 

 called Cheirotherium of Triassic times. The face goes by 

 degrees through the conditions seen in low sharks, am- 

 phibians and higher vertebrates. Step by step the in- 

 ternal structures advance toward their destined forms, 

 functions and positions. Thus, by a process of repeated 



