STAGES IN THE DEVELOPMENT 



GASTRULA 

 Stages 1 6 through 1 9 

 When the blastoderm has expanded to cover 

 about one-fourth of the yolk, there develops, 

 through mitotic activity, an accumulation of 

 cells at its margin (Stage 16) resulting in the 

 formation of the germ ring. In Stage 17 the 

 anterior lip of the blastopore is seen as an extra 

 accumulation of cells forming the embryonic 

 shield which becomes more prominent in Stages 

 18 and 19, seen best as viewed on the horizon 

 of the egg. 



NEURULA 



Stages 20 through 24 

 During Stages 20 to 24 the blastoderm con- 

 tinues to extend over the yolk. This culminates 

 in the closure of the blastopore late in Stage 24. 

 The blastoderm covers about half the yolk in 

 Stage 20. The embryonic shield in this stage 

 bears considerable resemblance to the open 

 neural plate of the amphibia. However, true 

 concrescence as in the amphibia does not occur. 

 There is a median longitudinal cellular pro- 

 liferation with a narrowing of the neural plate 

 resulting in the formation of a definitive pre- 

 somitic embryonic axis (Stage 21). After the 

 formation of the embryonic axis neurulation 

 proceeds rapidly. In Stage 22 the main divisions 

 of the brain can be discerned, the optic vesicle 

 is rudimentary, and the first somites appear. 

 There follows continued growth and further 

 differentiation of the divisions of the brain and 

 of the optic vesicle and a continuing addition of 

 somites in a caudal direction. In Stage 24 there 

 is a bilateral accumulation of cells, the rudiment 

 of the otic vesicle, at the level of the anterior 

 hind brain. 



TAIL BUD EMBRYO 



Stages 25 and 26 

 In Stage 25 (ca. 14 somites) the tail bud is well 

 denned. Around its periphery, the ectoderm is 

 directly continuous with that of the yolk sac. 



The blastopore is closed, but its location is ^^ 

 marked by a slight depression in the yolk. Al- ^F 

 though the main divisions of the brain can be 

 discerned, the brain ventricles are not yet 

 apparent. 



With continued growth, the tip of the tail 

 bud develops as a projection free of ectodermal 

 continuity with the yolk sac as in Stage 26 (ca. 17 

 somites). In this stage the pericardial sac is 

 developing and the heart rudiment forms as a 

 small bit of tissue in the pericardial sac under 

 the head. However, the heart rudiment does 

 not beat when it first forms. 



ORGANODIFFERENTIATION 

 Stages 27 through 53 



This period includes development extending 

 through to the complete absorption of the yolk. 

 The embryos prior to Stage 43, when hatching 

 normally occurs, were removed from the chori- 

 onic membranes for clearer observation and 

 illustration. 



In Stage 27, the heartbeat is just discernible ^& 

 in transmitted light. It is faint but regular. 

 Also, slow weak contractions of the anterior 

 somites result on gross mechanical stimulation 

 of the embryos. No cross striations are seen in 

 the developing muscle. 



In Stage 28, the heartbeat is still faint but 

 readily visible. Also there are spontaneous con- 

 tractions of the anterior somitic muscle. The 

 first formation of the brain ventricles is apparent 

 in the region of the hind brain indicated by the 

 formation of the roof of the fourth ventricle, 

 seen in the lateral view of the embryo. 



In Stage 29, muscle contractility is still limited 

 to the anterior somites. There has been an in- 

 crease in the size of the heart, but the circulation 

 of the blood is not established until Stage 30 

 and at first is sluggish and principally intraem- 

 bryonic. Sometimes at this stage there is seen 

 in the lower part of the otocyst an aggregation 

 of minute granules, the earliest indication of 

 the otoliths. 



