INTRODUCTION 341 



143C). The layer of blastomeres forms the blastoderm. The latter 

 e one cell in thickness, as in Amphioxiis (fig. MOT), or several cells 

 kness, as in the frog (fig. 143C). This hollow type of blastula often is 

 ;d to as a coeloblastula or blastosphere. However, in the gymnophionan 

 bia, the blastula departs from this vesicular condition and appears 

 >olid. The latter condition may be regarded as a stereoblastula, i.e., a 

 ilastula. A somewhat comparable condition is present in the bony ganoid 

 Amia and Lepisosteus. 



main characteristic of the blastula which does not possess auxiliary 

 is that the entire blastula is composed of formative cells, i.e., all the 

 nter directly into the formation of the embryo's body. 



2. Blastulae with Auxiliary or Trophoblast Tissue 



examination of those blastulae which possess auxiliary or trophoblast 

 shows a less simple condition than the round blastulae mentioned above, 

 first place, two types of cells are present, namely, formative cells which 

 into the composition of the embryonic body and auxiliary cells con- 

 mainly with trophoblast, or nutritional, functions. In the second place, 

 blastula which possesses auxiliary tissue, the latter often develops pre- 

 sly, that is, in advance of the formative cells of the blastula. As a 

 the arrangement of the formative cells into a configuration comparable 

 ; of those blastulae without trophoblast cells may be much retarded in 

 I instances. This condition is true particularly of the mammalian blastula 

 )cyst). 



lerally speaking, the blastulae which possess auxiliary tissue consist in 

 :arlier stages of a disc or a mass of formative cells at the peripheral 

 IS of which are attached the non-formative, auxiliary cells (fig. 159, 

 ierm-formative cells, periblast-non-formative; also figs. 145K, L; 147G, 

 he blastocoelic space lies below this disc of cells. However, in mammals 

 sciliary or nourishment-getting tissue tends to circumscribe the blastocoel, 

 is the formative cells occupy a polar area (fig. 145G, H). Blastulae, 

 ised of a disc-shaped mass of cells overlying a blastocoelic space, have 

 lescribed in classical terms as discoblastulae. 



3. Comparison of the Two Main Blastular Types 



e compare these two types of blastulae in terms of structure, it is evident 

 comparison is not logical unless the essential or formative cells and 

 irrangement are made the sole basis for the comparison, for only the 

 ive cells are common to both types of blastulae. To make the foregoing 

 ent more obvious, let us examine the essential structure of a typical 

 iastula, such as found in Amphioxus, as it is defined by the present- 

 nbryologist. 

 studies by Conklin, '32 and '33, demonstrated that the fertilized egg 



