then more probable that the mammal and bird possess this stage in their development simply because it has never been lost? Is not this a more reasonable view than to suppose that the gill slits of the embryos of the higher forms represent the adult gill slits of the fish that in some mysterious way have been pushed back into the embryo of the bird?
Fig. 11. Side views of head of embryo sharks, showing gill slits.
I could give many similar examples. All can be interpreted as embryonic survivals rather than as phyletic contractions. Not one of them calls for the latter interpretation.
The study of the cleavage pattern of the segmenting egg furnishes the most convincing evidence that a different explanation from the one stated in the biogenetic law is the more probable explanation.
Fig. 12. Cleavage stages of four types of eggs, showing the origin of the mesenchyme cells (stippled) and mesoderm cells (darker); a, Planarian; b, Annelid (Podarke); c, Mollusc (Crepidula), d, Mollusc (Unio).
It has been found that the cleavage pattern has the same general arrangement in the early stages of flat worms, annelids and molluscs (fig. 12). Obviously these stages have never been adult ancestors, and obviously if their resemblance has any meaning at all, it is that each group has retained the same general plan