188 THE AMERICAN NATURALIST. 
positively in what order at least four of the sets would be laid. 
His studies of ovarian eggs were, on the whole, excellent for 
his time, but his ideas in regard to yolk spheres differ consid- 
erably from those held to-day. 
For our knowledge of segmentation stages we are indebted 
to Agassiz alone. Although the segmentation stages drawn in 
his beautiful plates or described by him do not form a complete 
series, nevertheless they remain the only ones observed by 
reptilian embryologists. 
The next stage figured and described is that now generally 
known as the stage of the embryonic shield. The figures of 
this stage are beautiful and accurate, but what is now known 
to be the invagination of the primitive gut was mistaken for 
the beginning of the head development; in other words, the 
posterior end of the shield was mistaken for the anterior end. 
Many individuals and stages in the subsequent embryological 
development are figured, and helpful descriptions of surface 
views are given. Those figures showing the vitelline and 
allantoidian circulations deserve especial mention. 
The chapter and figures devoted to the development of 
organs contain much that is still useful to us. Indeed, his 
results are marvelous when we compare the methods of inves- 
tigators of that day with those of our own. For we must 
remember that in Agassiz’s time, imbedding in paraffin, the 
microtome, and, consequently, methods of reconstruction from 
sections were unknown. On the other hand, this absence of 
modern technique renders the chapter devoted to histology, 
however good for its time, of little scientific value for us to-day. 
Eight plates are devoted to figures of the eggs and recently 
hatched embryos of all the commoner North American species 
of turtles. So accurately are these drawn that one can with 
certainty identify the species from the egg or newly hatched 
young. These plates alone render the work indispensable. 
It is a tribute to the zeal and thoroughness of Agassiz and 
his helpers in this work that after forty years it stands to-day 
with its many unverified facts as an incentive to the reptilian 
embryologist to confirm and extend the work so magnificently 
begun. 
