IMPORTANCE OF THE BLASTULAR STAGE 351 



a naive analogy, nature does not build ten tracks to send ten trains with dif- 

 ferent destinies out of a sta:tion when she can use one track for all for at least 

 part of the way. So it is in development. A simple tubular heart appears in 

 all vertebrate embryos, followed by a simple, two-chambered* condition, not 

 because the two-chambered heart represents the recapitulated, two-chambered, 

 fish heart but rather because it, like the notochord, is a stage in a dynamic 

 developmental procedure of heart development in all vertebrates. As far as 

 the fish is concerned, when the common, two-chambered, rudimentary stage 

 of the heart is reached, nature shunts it off on a special track which develops 

 this simple, two-chambered condition into the highly muscular and efficient 

 two-chambered, adult heart adapted to the fish level of existence in its watery 

 environment. The three-chambered,* amphibian heart follows a similar pattern, 

 and it specializes at the three-chambered level because it fits into the amphibian 

 way of life. So it is with the embryonic pharyngeal area with its visceral and 

 aortal arches which resemble one another throughout the vertebrate group 

 during early embryonic development. The elaboration of a common, pha- 

 ryngeal area with striking resemblances throughout the vertebrate group can 

 be explained more easily and rationally on the assumption that it represents 

 a common, physiologically important step in a developmental procedure. 



This general view suggests the conclusion that ontogeny tends to use com- 

 mon developmental methods wherever and whenever these methods can be 

 utilized in the development of a large group of animals. Development or 

 ontogeny, therefore, recapitulates phylogenetic procedures and not adult mor- 

 phological stages. One explanation for this conservation of effort may be 

 that, physiologically speaking, the number of essential methods, whereby a 

 specific end may be produced, probably is Hmited. Another explanation sug- 

 gests that an efficient method never is discarded. 



F. Importance of the Blastular Stage in Embryonic Development 



Superficially in many forms, chordate and non-chordate, the blastula is a 

 hollow, rounded structure containing the blastocoelic space within. It is tempt- 

 ing to visualize this form as the basic, essential form of the blastula. How- 

 ever, the so-called blastular stage in reality presents many forms throughout 

 the animal kingdom, some solid, some round and hollow, and others in the 

 form of a flattened disc or even an elongated band. Regardless of their shape, 

 all blastulae have this in common: they represent an association of pre- 

 sumptive organ-forming areas, areas which later move to new positions in 

 the forming body, increase in cellular mass, and eventually become molded 

 into definite structures. One of the main purposes of blastulation, therefore, 

 may be stated as the elaboration (or establishment) of the major, presumptive 

 organ-forming areas of the particular species and their arrangement in a 

 particular pattern which permits their ready manipulation during the next 



* Exclusive of the sinus venosus. 



